diff --git a/day21/common/monkey2.go b/day21/common/monkey2.go
new file mode 100644
index 0000000..15493a3
--- /dev/null
+++ b/day21/common/monkey2.go
@@ -0,0 +1,228 @@
+package common
+
+import (
+	"bufio"
+	"strconv"
+	"strings"
+)
+
+
+type Monkey interface {
+  Apply() int
+  Invert(int) int
+  IsHuman() bool
+}
+
+
+type Human struct {
+  name string
+  number int
+}
+
+func (monkey *Human) Apply() int {
+  return monkey.number
+}
+
+func (monkey *Human) Invert(total int) int {
+  return total
+}
+
+func (monkey *Human) IsHuman() bool {
+  return true
+}
+
+
+type NumberMonkey struct {
+  name string
+  number int
+}
+
+func (monkey *NumberMonkey) Apply() int {
+  return monkey.number
+}
+
+func (monkey *NumberMonkey) Invert(total int) int {
+  return total - monkey.number
+}
+
+func (monkey *NumberMonkey) IsHuman() bool {
+  return false
+}
+
+
+type PlusMonkey struct {
+  name string
+  operand1 Monkey
+  operand2 Monkey
+}
+
+func (monkey *PlusMonkey) Apply() int {
+  return monkey.operand1.Apply() + monkey.operand2.Apply()
+}
+
+func (monkey *PlusMonkey) Invert(total int) int {
+  if monkey.operand1.IsHuman() {
+    return monkey.operand1.Invert(total - monkey.operand2.Apply())
+  } else {
+    return monkey.operand2.Invert(total - monkey.operand1.Apply())
+  }
+}
+
+func (monkey *PlusMonkey) IsHuman() bool {
+  return monkey.operand1.IsHuman() || monkey.operand2.IsHuman()
+}
+
+
+type MinusMonkey struct {
+  name string
+  operand1 Monkey
+  operand2 Monkey
+}
+
+func (monkey *MinusMonkey) Apply() int {
+  return monkey.operand1.Apply() - monkey.operand2.Apply()
+}
+
+func (monkey *MinusMonkey) Invert(total int) int {
+  if monkey.operand1.IsHuman() {
+    return monkey.operand1.Invert(total + monkey.operand2.Apply())
+  } else {
+    return monkey.operand2.Invert(monkey.operand1.Apply() - total)
+  }
+}
+
+func (monkey *MinusMonkey) IsHuman() bool {
+  return monkey.operand1.IsHuman() || monkey.operand2.IsHuman()
+}
+
+
+type TimesMonkey struct {
+  name string
+  operand1 Monkey
+  operand2 Monkey
+}
+
+func (monkey *TimesMonkey) Apply() int {
+  return monkey.operand1.Apply() * monkey.operand2.Apply()
+}
+
+func (monkey *TimesMonkey) Invert(total int) int {
+  if monkey.operand1.IsHuman() {
+    return monkey.operand1.Invert(total / monkey.operand2.Apply())
+  } else {
+    return monkey.operand2.Invert(total / monkey.operand1.Apply())
+  }
+}
+
+func (monkey *TimesMonkey) IsHuman() bool {
+  return monkey.operand1.IsHuman() || monkey.operand2.IsHuman()
+}
+
+
+type DivisionMonkey struct {
+  name string
+  operand1 Monkey
+  operand2 Monkey
+}
+
+func (monkey *DivisionMonkey) Apply() int {
+  return monkey.operand1.Apply() / monkey.operand2.Apply()
+}
+
+func (monkey *DivisionMonkey) Invert(total int) int {
+  if monkey.operand1.IsHuman() {
+    return monkey.operand1.Invert(total * monkey.operand2.Apply())
+  } else {
+    return monkey.operand2.Invert(monkey.operand1.Apply() / total)
+  }
+}
+
+func (monkey *DivisionMonkey) IsHuman() bool {
+  return monkey.operand1.IsHuman() || monkey.operand2.IsHuman()
+}
+
+
+type EqualityMonkey struct {
+  name string
+  operand1 Monkey
+  operand2 Monkey
+}
+
+func (monkey *EqualityMonkey) Apply() int {
+  if monkey.operand1.Apply() == monkey.operand2.Apply() {
+    return 1
+  } else {
+    return 0
+  }
+}
+
+func (monkey *EqualityMonkey) Invert(total int) int {
+  if monkey.operand1.IsHuman() {
+    return monkey.operand1.Invert(monkey.operand2.Apply() - total)
+  } else {
+    return monkey.operand2.Invert(monkey.operand1.Apply() - total)
+  }
+}
+
+func (monkey *EqualityMonkey) IsHuman() bool {
+  return monkey.operand1.IsHuman() || monkey.operand2.IsHuman()
+}
+
+
+type StringCouple struct {
+  left string
+  right string
+}
+
+
+type UnresolvedMonkey struct {
+  name string
+  symbol string
+}
+
+
+func Parse(scanner bufio.Scanner, rootIsEquality bool) Monkey {
+  unresolvedMonkeys := make(map[StringCouple]UnresolvedMonkey)
+  resolvedMonkeys := make(map[string]Monkey)
+
+  for scanner.Scan() {
+    line := scanner.Text()
+    splittedLine := strings.Split(line, " ")
+    if len(splittedLine) == 2 {
+      number, _ := strconv.Atoi(splittedLine[1])
+      if splittedLine[0][:4] == "humn" {
+        resolvedMonkeys[splittedLine[0][:4]] = &Human{splittedLine[0][:4], number}
+      } else {
+        resolvedMonkeys[splittedLine[0][:4]] = &NumberMonkey{splittedLine[0][:4], number}
+      }
+    } else {
+      unresolvedMonkeys[StringCouple{splittedLine[1], splittedLine[3]}] = UnresolvedMonkey{splittedLine[0][:4], splittedLine[2]}
+    }
+  }
+
+  for len(unresolvedMonkeys) > 0 {
+    for monkeyCouple := range(unresolvedMonkeys) {
+      monkey1, ok1 := resolvedMonkeys[monkeyCouple.left]
+      monkey2, ok2 := resolvedMonkeys[monkeyCouple.right]
+      if ok1 && ok2 {
+        if rootIsEquality && unresolvedMonkeys[monkeyCouple].name == "root" {
+          resolvedMonkeys[unresolvedMonkeys[monkeyCouple].name] = &EqualityMonkey{unresolvedMonkeys[monkeyCouple].name, monkey1, monkey2}
+        } else {
+          switch unresolvedMonkeys[monkeyCouple].symbol {
+            case "+":
+              resolvedMonkeys[unresolvedMonkeys[monkeyCouple].name] = &PlusMonkey{unresolvedMonkeys[monkeyCouple].name, monkey1, monkey2}
+            case "-":
+              resolvedMonkeys[unresolvedMonkeys[monkeyCouple].name] = &MinusMonkey{unresolvedMonkeys[monkeyCouple].name, monkey1, monkey2}
+            case "*":
+              resolvedMonkeys[unresolvedMonkeys[monkeyCouple].name] = &TimesMonkey{unresolvedMonkeys[monkeyCouple].name, monkey1, monkey2}
+            case "/":
+              resolvedMonkeys[unresolvedMonkeys[monkeyCouple].name] = &DivisionMonkey{unresolvedMonkeys[monkeyCouple].name, monkey1, monkey2}
+          }
+        }
+        delete(unresolvedMonkeys, monkeyCouple)
+      }
+    }
+  }
+
+  return resolvedMonkeys["root"]
+}
diff --git a/day21/ex1/main.go b/day21/ex1/main.go
new file mode 100644
index 0000000..ffc68a7
--- /dev/null
+++ b/day21/ex1/main.go
@@ -0,0 +1,14 @@
+package main
+
+import (
+  "aoc2022/day21/common"
+  "bufio"
+  "fmt"
+  "os"
+)
+
+func main() {
+  rootMonkey := common.Parse(*bufio.NewScanner(os.Stdin), false)
+
+  fmt.Println(rootMonkey.Apply())
+}
diff --git a/day21/ex2/main.go b/day21/ex2/main.go
new file mode 100644
index 0000000..ca4eb72
--- /dev/null
+++ b/day21/ex2/main.go
@@ -0,0 +1,14 @@
+package main
+
+import (
+  "aoc2022/day21/common"
+  "bufio"
+  "fmt"
+  "os"
+)
+
+func main() {
+  rootMonkey := common.Parse(*bufio.NewScanner(os.Stdin), true)
+
+  fmt.Println(rootMonkey.Invert(0))
+}