package main

import (
	"fmt"
)

func main() {
	var n, t int
	var s string
	var m = make(map[string]int)

	fmt.Scanf("%d %d", &n, &t)
	fmt.Scan(&s)

	for i := 0; i <= len(s)-t; i ++ {
		key := s[i : i+t]
		m[key] ++
		if v, _ := m[key]; v >= 2 {
			fmt.Print("YES")
		}
	}

	fmt.Print("NO")
}