Решение задачи Простой ряд с Acmp

#include <bits/stdc++.h>
using namespace std;
 
bool isPrime(unsigned long a)
{
    bool t = true;
    for(unsigned long i = 2; i <= sqrt(a); ++i)
        if(!(a % i)) {
            t = false; break;
        }
    return t;
}
 
int main()
{
    unsigned long m,n;
    int s = 0, s1 = 0,k = 0,d = 0;
    cin >> n;
    string ss;
    vector<int> b;
    b.clear();
    ss.erase();
    vector<int> a;
    for(unsigned long i = 0; i <= 21000; ++i){
        if(isPrime(i)){
            d = i;
            while(d){
                a.push_back(d % 10);
                d/=10;
            }
            for(int j = a.size() - 1; j >= 0; --j)
                ss.push_back(a[j] + 48);
            a.clear();
        }}
    for(int i = 0; i < n; ++i){
        cin >> m;
        cout << ss[m];
    }
    return 0;
}