foo.c: In function 'main':
foo.c:6:36: error: 'mod' undeclared (first use in this function)
    6 |         long long int answer = a11%mod * a22%mod - a12%mod * a21%mod;
      |                                    ^~~
foo.c:6:36: note: each undeclared identifier is reported only once for each function it appears in

[Hydro](https://hydro.ac)提供评测服务