diff --git a/du4/program.c b/du4/program.c
index ce9b0b2..918eb39 100644
--- a/du4/program.c
+++ b/du4/program.c
@@ -61,6 +61,9 @@ int check(char *str){
         if(str[i]=='='){
             flag=1;
         }
+        if((str[i]>='a'&&str[i]<='z')||(str[i]>='A'&&str[i]<='Z')){
+            return 0;
+        }
     }
     return flag==1?1:0;
 }