diff --git a/du3/program.c b/du3/program.c
index a7586b0..44c1d2d 100644
--- a/du3/program.c
+++ b/du3/program.c
@@ -17,7 +17,7 @@ void init(struct stack *s){
 }
 
 int is_empty(struct stack *s){return s->size==0;}
-int is_full(struct stack *s){return s->size==STACK_SIZE-1;}
+int is_full(struct stack *s){return s->size==STACK_SIZE;}
 
 
 void push(struct stack *s, float value){