foo.cc: In function ‘int main()’:
foo.cc:20:25: warning: statement has no effect [-Wunused-value]
   20 |                     for(i;i<=n1;i++)
      |                         ^
foo.cc:21:40: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
   21 |                     if(i%400==0||i%4==0&&i%100!=0) s++;
      |                                  ~~~~~~^~~~~~~~~~