package main

import (
    "fmt"
    "io"
    "os"
    "strings"
)

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

    in, _ := os.Open("input.txt")
    out, _ := os.Create("output.txt")

    defer in.Close()
    defer out.Close()

    fmt.Fscanf(in, "%d %d", &n, &t)

    s := ""
    for {
        _, err := fmt.Fscanln(in, &temp)
        if err == io.EOF {
            break
        }
        s += strings.TrimSpace(temp)
    }

    if len(s) != n {
        fmt.Fprint(out, "ERROR")
        return
    }

    for i := 0; i <= len(s)-t; i ++ {
        key := s[i : i+t]
        if _, ok := m[key]; !ok {
            m[key] = 0
        }
        m[key] ++
    }

    for _, v := range m {
        if v >= 2 {
            fmt.Fprint(out, "YES")
            return
        }
    }
    fmt.Fprint(out, "NO")
}