diff --git a/slides/cours_11.md b/slides/cours_11.md
index 63be99a02861c4f63cffedc3ed2dbb79dfd92213..d4ac7a03e634febafd6870551fac2162c0d446fd 100644
--- a/slides/cours_11.md
+++ b/slides/cours_11.md
@@ -584,7 +584,7 @@ typedef struct _queue {
 ```C
 void queue_enqueue(queue *fa, int val) {
     if ((fa->head == 0 && fa->tail == fa->capacity-1) ||
-            (fa->tail == (fa->head-1) % (fa->capacity-1))) {
+            (fa->tail == (fa->head - 1 + fa->capacity) % fa->capacity) {
         return; // queue is full
     }
     if (fa->head == -1) { // queue was empty