diff --git a/slides/cours_21.md b/slides/cours_21.md
index 3b711ea77aea8edff687352a9d21b79607b6d04d..e99cb775bc98d0165cafef6ab52e34e8bf83af51 100644
--- a/slides/cours_21.md
+++ b/slides/cours_21.md
@@ -371,7 +371,7 @@ int depth(node *qt) {
 
 ```C
 noeud position(li, co, arbre)
-    d = profondeur(arbre);
+    d = profondeur(arbre) - 1;
     tant_que (d >= 1)
         index = 2 * ((li % 2^d) / 2^(d-1))
             + (col % 2^d) / 2^(d-1)