--- a/counter.c
+++ b/counter.c
@@ -25,7 +25,9 @@
 
     start = time_str->tm_hour * 60 * 60 + time_str->tm_min * 60 + time_str->tm_sec;
 
-    system (argv[1]);
+    if (system (argv[1]) == -1) {
+        return EXIT_FAILURE;
+    }
 
     time (&t);
     time_str = gmtime (&t);