foo.cc: In function 'int main()':
foo.cc:43:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d%d", &n, &m);
     ~~~~~^~~~~~~~~~~~~~~~
foo.cc:46:14: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         scanf("%d%d", &x, &y);
         ~~~~~^~~~~~~~~~~~~~~~
foo.cc:63:26: warning: 'k' may be used uninitialized in this function [-Wmaybe-uninitialized]
     if(num >= 2 || f[k][0] == 1) printf("-1");
                    ~~~~~~^

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。