TOI10: จำนวนเฉพาะ (Prime)

#include <bits/stdc++.h>

using namespace std;

int qs[10000001];
vector <bool> prime(10000001, true);

int main(){

    int n;
    scanf("%d", &n);

    for(int i = 2; i <= 10000000; i ++){
        if(prime[i]){
            for(int j = i + i; j <= 10000000; j = j + i){
                prime[j] = false;
            }
        }
        qs[i] = qs[i-1] + prime[i];
        if(qs[i] == n){
            printf("%d", i);
            return 0;
        }
    }

    return 0;
}