diff --git a/slides/cours_15.md b/slides/cours_15.md
index 4b1d29228ea3a4bee26480e9e7e1ab8eff1cab4f..09a8ed06a9c28de6eb60a4ad216f6dbdbddf54a6 100644
--- a/slides/cours_15.md
+++ b/slides/cours_15.md
@@ -707,7 +707,7 @@ void pretty_print(tree_t tree, int n) {
             printf(" ");
         }
         printf("%d\n", tree->info);
-        pretty_print(tree->left);
+        pretty_print(tree->left, n+1);
     }
 }
 ```