решето Эратосфена

#include <iostream>

#include <vector>

using namespace  std;
int main()
{
        int k, x = 1;
        cin >> k;
        int n=16 * k;
        vector<bool> 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;
        for (int i = 2; i < n;++i) {
            if (prime[i]) {
                if (x++ != k) continue;
            else {
                cout << i;
            }

           }

        }
}