D. Sweety Miya The Saver- solve by Shanto

/*    BISMILLAHIR-RAHMANIR-RAHIM
 ____________________________________
|                                    |
|      SHANTO_SUST_SWE-19__029       |
|      shanto-swe029.github.io       |
|____________________________________|
*/

#include <bits/stdc++.h>
using namespace std;

#define M 10000
int marked[M/64 + 2];

#define on(x) (marked[x/64] & (1<<((x%64)/2)))
#define mark(x)  marked[x/64] |= (1<<((x%64)/2))

void sieve(int n) {
  for (int i = 3; i * i < n; i += 2) {
    if (!on(i)) {
      for (int j = i * i; j <= n; j += i + i) {
        mark(j);
      }
    }
  }
}

bool isPrime(int num) {
  return num > 1 && (num == 2 || ((num & 1) && !on(num)));
}

std::vector < string > primes;

void createPrimeVector()
{
    for(int i = 1; i < 10000; i++) {
        if(isPrime(i)) {
            primes.push_back(to_string(i));
        }
    }
}


int main()
{
    sieve(10000);
    createPrimeVector();

    int T, t = 0;
    std::cin >> T;
    int n, sz = primes.size() - 1;
    string s, y;
    while(T--) {
        std::cout << "Case " << ++t << ": ";
        std::cin >> n >> s;
        bool found = false;
        for(int i = sz; i >= 0; i--) {
            if (s.find(primes[i]) != string::npos) {
                found = true;
                y = primes[i];
                break;
            }
        }
        if(found) {
            cout << y << endl;
        }
        else {
            cout << "This is a junk!" << endl;
        }
    }

    return 0;
}

//ALHAMDULILLAH