opmat.cpp: In function 'int main()': opmat.cpp:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result opmat.cpp:13: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result opmat.cpp:14: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result opmat.cpp:17: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result opmat.cpp:21: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result opmat.cpp:25: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result