/in/foo.cc: In function 'int main()':
/in/foo.cc:6:7: warning: statement has no effect [-Wunused-value]
  for(i;i<=n;i++){
       ^
/in/foo.cc:4:18: warning: unused variable 'a' [-Wunused-variable]
  int n , i = 1 , a = 1;
                  ^