123

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int n, k, temp, i = 0;
        cin >> n >> k;
    string s, c = "";
        cin >> s;

    while(c[k - 1] != s[n - 1])
    {
        c = s.substr(i, k);
        temp = s.find(c);
        ///cout << "c = " << c << "   " << "temp = " << temp << "   " <<  "i = " << i << endl;
        if(temp != i) {cout << "YES"; return 0;}
        i++;
    }
    cout << "NO";
}