Compilare: user.cpp: In function ‘int sieve(int)’: user.cpp:9:22: warning: operation on ‘i’ may be undefined [-Wsequence-point] for(i = 1; ((i * ++i) << 1) < n - 3; ++i) ^~~ user.cpp: In function ‘int main()’: user.cpp:41:35: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("ciur.in", "r", stdin); ^ user.cpp:42:37: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("ciur.out", "w", stdout); ^ user.cpp:44:27: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result] int N; scanf("%d", &N); ^