Compilare: user.cpp: In function ‘void back(int, long long int)’: user.cpp:40:28: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (pos == primes.size()) ^ user.cpp: In function ‘void read_data()’: user.cpp:81:39: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("armonica.in", "r", stdin); ^ user.cpp:82:20: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] scanf("%d", &n); ^