diff --git a/HumanEvalLean/HumanEval102.lean b/HumanEvalLean/HumanEval102.lean
index 7211b4b..4bdf02d 100644
--- a/HumanEvalLean/HumanEval102.lean
+++ b/HumanEvalLean/HumanEval102.lean
@@ -1,5 +1,69 @@
-def choose_num : Unit :=
-  ()
+def Nat.isEven (n : Nat) : Bool :=
+  n % 2 = 0
+
+def chooseNum (lo hi : Nat) : Int :=
+  match compare lo hi, hi.isEven with
+  | .lt, true | .eq, true => hi
+  | .lt, false            => hi - 1
+  | _, _                  => -1
+
+example : chooseNum 12 15 = 14       := rfl
+example : chooseNum 13 12 = -1       := rfl
+example : chooseNum 12 15 = 14       := rfl
+example : chooseNum 13 12 = -1       := rfl
+example : chooseNum 33 12354 = 12354 := rfl
+example : chooseNum 5234 5233 = -1   := rfl
+example : chooseNum 6 29 = 28        := rfl
+example : chooseNum 27 10 = -1       := rfl
+example : chooseNum 7 7 = -1         := rfl
+example : chooseNum 546 546 = 546    := rfl
+
+macro "chooseNum_cases" lo:ident hi:ident : tactic => `(tactic|(
+  cases _ : compare $lo $hi <;> cases _ : $(hi).isEven <;> simp only [chooseNum, *] at *
+))
+
+macro "chooseNum_trivial" : tactic => `(tactic|(
+  simp only [(· ∈ ·), Nat.isEven, Nat.compare_eq_eq, Nat.compare_eq_lt, Nat.compare_eq_gt,
+             decide_eq_false_iff_not, decide_eq_true_eq] at *
+  omega
+))
+
+namespace Std.Range
+
+-- A specification for `m` being the even maximum of range `r`.
+structure EvenMax (r : Range) (m : Nat) : Prop where
+  mem      : m ∈ r                     := by chooseNum_trivial
+  even     : m.isEven                  := by chooseNum_trivial
+  max_even : ∀ n ∈ r, n.isEven → n ≤ m := by chooseNum_trivial
+
+namespace EvenMax
+
+theorem unique (h₁ : [lo:hi].EvenMax m₁) (h₂ : [lo:hi].EvenMax m₂) : m₁ = m₂ :=
+  Nat.le_antisymm (h₂.max_even _ h₁.mem h₁.even) (h₁.max_even _ h₂.mem h₂.even)
+
+theorem to_chooseNum (h : [lo:hi + 1].EvenMax m) : chooseNum lo hi = m := by
+  have ⟨_, _, _⟩ := h
+  chooseNum_cases lo hi <;> try chooseNum_trivial
+  · have : [lo:hi + 1].EvenMax (hi - 1) := { }
+    simp only [h.unique this, Nat.isEven, decide_eq_true_eq, decide_eq_false_iff_not] at *
+    omega
+  · rw [h.unique { : [lo:hi + 1].EvenMax hi }]
+
+-- A given number `m` is the even maximum of the range `{lo, ..., hi}` iff `chooseNum` says so.
+theorem iff_chooseNum : [lo:hi + 1].EvenMax m ↔ chooseNum lo hi = m where
+  mp    := EvenMax.to_chooseNum
+  mpr h := by constructor <;> chooseNum_cases lo hi <;> chooseNum_trivial
+
+-- There does not exist an even maximum of the range `{lo, ..., hi}` iff `chooseNum` returns `-1`.
+theorem not_iff_chooseNum : ¬(∃ m, [lo:hi + 1].EvenMax m) ↔ (chooseNum lo hi = -1) where
+  mpr h := fun ⟨_, ⟨_, _, _⟩⟩ => by chooseNum_cases lo hi <;> chooseNum_trivial
+  mp h  := by
+    chooseNum_cases lo hi <;> exfalso
+    · exact h ⟨hi - 1, {}⟩
+    · exact h ⟨hi, {}⟩
+    · exact h ⟨hi, {}⟩
+
+end Std.Range.EvenMax
 
 /-!
 ## Prompt
@@ -8,7 +72,7 @@ def choose_num : Unit :=
 
 def choose_num(x, y):
     """This function takes two positive numbers x and y and returns the
-    biggest even integer number that is in the range [x, y] inclusive. If 
+    biggest even integer number that is in the range [x, y] inclusive. If
     there's no such number, then the function should return -1.
 
     For example:
@@ -47,4 +111,4 @@ def check(candidate):
     assert candidate(546, 546) == 546
 
 ```
--/
\ No newline at end of file
+-/