Compilare: user.cpp: In function ‘int main()’: user.cpp:15:38: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("gravity.in", "r", stdin); ^ user.cpp:16:40: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("gravity.out", "w", stdout); ^ user.cpp:19:28: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] scanf("%d%d\n", &N, &M); ^ user.cpp:21:23: warning: ignoring return value of ‘char* gets(char*)’, declared with attribute warn_unused_result [-Wunused-result] gets(a[i] + 1); ^ In file included from /usr/include/stdio.h:934:0, from /usr/include/c++/4.8/cstdio:42, from /usr/include/i386-linux-gnu/c++/4.8/bits/stdc++.h:46, from user.cpp:1: In function ‘char* gets(char*)’, inlined from ‘int main()’ at user.cpp:21:23: /usr/include/i386-linux-gnu/bits/stdio2.h:230:28: warning: call to ‘__gets_warn’ declared with attribute warning: please use fgets or getline instead, gets can't specify buffer size [enabled by default] return __gets_warn (__str); ^ /tmp/ccEvLHBr.o: In function `main': user.cpp:(.text.startup+0x83): warning: the `gets' function is dangerous and should not be used.