1152 Google Recruitment (20 分)

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

#define debug puts("YES");
#define rep(x,y,z) for(int (x)=(y);(x)<(z);(x)++)
#define ll long long
#define ull unsigned long long

#define lrt int l,int r,int rt
#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1
#define root l,r,rt
#define mst(a,b) memset((a),(b),sizeof(a))
#define pii pair<int,int>
#define piii pair<int,pii>
#define fi first                                  
#define se second
#define mk(x,y) make_pair(x,y)

#define base 2333
const int mod=1e9+7;
const int maxn=1e4+10;
const int ub=1e6;
ll powmod(ll x,ll y){ll t; for(t=1;y;y>>=1,x=x*x%mod) if(y&1) t=t*x%mod; return t;}
ll gcd(ll x,ll y){
    if(y==0) return x;
    return gcd(y,x%y);
}
int n,k;
string s;
int trans(int l,int r){
    int ret=0;
    rep(i,l,r+1) ret=ret*10+(s[i]-'0');
    return ret;
}
int judge(int x){
    if(x==2) return 1;
    if(x%2==0||x==1) return 0;
    for(int i=2;1LL*i*i<=x;i++)
        if(x%i==0) return 0;
    return 1;
}
int main(){
    cin>>n>>k;
    cin>>s;
    int len=s.size(),ans=-1;
    for(int i=k-1;i<len;i++){
        int xx=trans(i-k+1,i);
        if(judge(xx)){
            ans=i;
            break;
        }
    }
    if(ans==-1) cout<<"404"<<"\n";
    else{
        rep(i,ans-k+1,ans+1) 
            cout<<s[i];
        cout<<"\n";
    }
    return 0; 
}