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);
~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
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