#include <cstdio>
#include <cstdlib>
#include <map>
#include <algorithm>
using namespace std;

#define Mod 66587
#define Nmax 16400

int C[Nmax], P[Nmax];
int N, K;
char S[Nmax];
map<int, int> Hash;

bool Check(int L)
{
    Hash.clear();
    for(int i = 1; i + L - 1 <= N; ++ i)
    {
        int SubC = (C[i + L - 1] - C[i - 1] * P[L] + Mod) % Mod;

        Hash[SubC] ++;
        if(Hash[SubC] >= K) return 1;
    }
    return 0;
}

int main()
{
    freopen("substr.in", "r", stdin);
    freopen("substr.out", "w", stdout);
    int i;
    scanf("%i %i\n", &N, &K);
    gets(S + 1);
    P[0] = 1;
    for(i = 1; i <= N; ++ i)
        P[i] = (P[i - 1] * 257) % Mod;

    for(i = 1; i <= N; ++ i)
        C[i] = (C[i - 1] * 257 + S[i]) % Mod;

    int Left = 0, Right = N, Mid, Ans = -1;
    while(Left <= Right)
    {
        Mid = (Left + Right) / 2;
        if(Check(Mid)) Ans = Mid, Left = Mid + 1;
        else Right = Mid - 1;
    }
    printf("%i\n", Ans);
    return 0;
}