diff --git a/du4/program.c b/du4/program.c
index 152c4c7..b1bf4b0 100644
--- a/du4/program.c
+++ b/du4/program.c
@@ -48,9 +48,12 @@ double compare_percent(double a, double b, double eps)
 }
 
 int check(char *str){
+    char symb[5]="*-+/=";
     for(int i =0;i<strlen(str);i++){
-        if(str[i]=='='&&str[i+1]=='='){
-            return 0;
+        for(int k =0;k<5;k++){
+            if(str[i]==symb[k]&&str[i+1]==symb[k]){
+                return 0;
+            }
         }
     }
     return 1;