https://pintia.cn/problem-sets/994805260223102976/problems/1071785997033074688
#include <iostream>
#include <string>
#include <cmath>
using namespace std;
bool isp(int num){
for(int i = 2; i <= sqrt(num); i++)
if(!(num%i))
return false;
return true;
};
int main(){
int L, K, flag = 0, len;
string s, num;
cin >> L >> K >> s;
for(int i = 0; i < L; i++){
len = 0;
num = "";
for(int j = i; j < i+K && j < L; j++){
len++;
num += s[j];
if(len == K && isp(stoi(num)))
flag = 1;
}
if(flag){
cout << num;
break;
}
}
if(!flag) cout << "404";
return 0;
}