diff --git a/test.c b/test.c
index 0118cc8..56ed6a4 100644
--- a/test.c
+++ b/test.c
@@ -5,7 +5,7 @@
 
 int sum(int n)
 {
-    return ((n + 1) * n) / 2;
+    return ((n - 1) * n) / 2;
 }
 
 void usage(const char* app_name, const char* err_string)