Решето эратосфена

#include <iostream>

using namespace std;

int main() {

    cin >> n;

    was[0] = was[1] = true;

    for (int i = 2; i <= 10000000; i++) {
        if (was[i] != true) {
            if (i * 1ll * i <= 10000000) {
                for (int j = i * i; j <= 10000000; j += i)
                    was[j] = true;
            }
        }

    }

    return 0;

}