diff --git a/dm5/lib/lib.ml b/dm5/lib/lib.ml
index 2ffe31a..b75db38 100644
--- a/dm5/lib/lib.ml
+++ b/dm5/lib/lib.ml
@@ -4,7 +4,7 @@ let zip ta tb = Array.map2 (fun a b -> a,b) ta tb;;
 (* Exercice 1 *)
 let tirette t =
   let n = Array.length t in
-  Array.init (n/2) (fun i -> t.(i) + t.(n-i))
+  Array.init (n/2) (fun i -> t.(i) + t.(n-i-1))
 ;;
 
 (* Exercice 2 *)