I have found the code from MAXimal

```
int n;
vector<char> prime (n+1, true);
prime[0] = prime[1] = false;
for (int i=2; i<=n; ++i)
if (prime[i])
if (i * 1ll * i <= n)
for (int j=i*i; j<=n; j+=i)
prime[j] = false;
```

why they use multiplication by 111 on 6th line of code