foo.c: In function 'main':
foo.c:6:37: error: 'mod' undeclared (first use in this function)
    6 |         long long int answer = (a11%mod) * (a22%mod) - (a12%mod) * (a21%mod);
      |                                     ^~~
foo.c:6:37: note: each undeclared identifier is reported only once for each function it appears in

[Hydro](https://hydro.ac)提供评测服务