foo.cc: In function ‘int main()’:
foo.cc:22:32: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
   22 |                         if(flag=0){
      |                            ~~~~^~
foo.cc:23:38: error: expected primary-expression before ‘;’ token
   23 |                                 S=S+!;
      |                                      ^