prefix.cpp: In function 'int main()': prefix.cpp:8: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result prefix.cpp:8: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result prefix.cpp:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result prefix.cpp:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result prefix.cpp:20: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result