foo.cc: In function 'void fre()':
foo.cc:12:29: warning: ignoring return value of 'int system(const char*)', declared with attribute warn_unused_result [-Wunused-result]
 void fre() { system("clear"), freopen("A.txt", "r", stdin); freopen("Ans.txt","w",stdout); }
              ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:12:29: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
foo.cc:12:68: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
 void fre() { system("clear"), freopen("A.txt", "r", stdin); freopen("Ans.txt","w",stdout); }
                                                             ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
foo.cc: In function 'void Fre()':
foo.cc:13:29: warning: ignoring return value of 'int system(const char*)', declared with attribute warn_unused_result [-Wunused-result]
 void Fre() { system("clear"), freopen("A.txt", "r", stdin);}
              ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:13:29: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
foo.cc: In function 'int main()':
foo.cc:110:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     sc("%d", &n);
       ^
foo.cc:114:11: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         sc("%d %d", &u, &v);
           ^
foo.cc:123:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     sc("%d", &m);
       ^
foo.cc:128:11: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         sc("%d %d", &x, &k);
           ^

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