diff --git a/cv3/program.c b/cv3/program.c
index 3bad3d7..799a97b 100644
--- a/cv3/program.c
+++ b/cv3/program.c
@@ -81,7 +81,11 @@ int main(int argc, char const *argv[])
         {
             break;
         }
-
+        if(isalpha(buf) != 0)
+        {
+            printf("bad input\n");
+            return 0;
+        }
         buf[strcspn(buf, "\n")] = '\0';
         if(!check_op(buf[0]))
         {