foo.cc: In function 'int main()':
foo.cc:217:8: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
 freopen("INPUT.txt", "r", stdin);
 ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:218:8: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
 freopen("OUTPUT.txt", "w", stdout);
 ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务