/in/foo.cc: In function 'int main()':
/in/foo.cc:20:28: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
  freopen("xx.in","r",stdin); freopen("xx.out","w",stdout);
                            ^
/in/foo.cc:20:58: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
  freopen("xx.in","r",stdin); freopen("xx.out","w",stdout);
                                                          ^
/in/foo.cc:22:25: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d\n",& x,& y);
                         ^
/in/foo.cc:28:14: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   scanf("\n");
              ^