【PAT甲级 素数判断】1152 Google Recruitment (20 分)

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

int N, K;
string num;

bool isPrime(long long n) {
    
    
    int sq = sqrt(n);
    for(int i = 2;i <= sq;++i)
        if(n % i == 0)
            return false;
    return true;
}

int main() {
    
    
    cin >> N >> K;
    cin >> num;
    
    int i;
    string subNum;
    for(i = 0;i <= N - K;++i){
    
    
        subNum = num.substr(i, K);
        if( isPrime(stoll(subNum)) ) break;
    }
    i == N-K+1 ? cout<<404 : cout<<subNum;
    
    return 0;
}