#include <fstream>
#include <cstring>
#include <algorithm>
#define MAXN 17000
#define MAXLG 20
using namespace std;
ifstream fin("substr.in");
ofstream fout("substr.out");

char A[MAXN];
struct entry{
    int nr[2],p;
} L[MAXN];
int P[MAXN][MAXLG], N, i ,stp ,cnt, K;
int Sol;

bool cmp(const entry &a, const entry &b){
    return a.nr[0]==b.nr[0] ? (a.nr[1] < b.nr[1]) : (a.nr[0] < b.nr[0]);
}

int lcp(int x, int y) {
    int k, ret = 0;
    if (x == y) return N - x;
    for (k = stp - 1; k >= 0 && x < N && y < N; --k)
        if (P[k][x] == P[k][y])
            x += 1 << k, y += 1 << k, ret += 1 << k;
    return ret;
}

int main(){

    fin >> N >> K;

    fin >> A;

    for(int i=0;i<N;i++)
        P[0][i] = A[i] - 'a';
    for(stp = 1, cnt = 1; cnt >> 1 < N; ++stp, cnt <<=1){
        for(i=0;i<N;i++){
            L[i].nr[0] = P[stp-1][i];
            L[i].nr[1] = i + cnt < N ? P[stp-1][i + cnt] : 1;
            L[i].p = i;
        }
        sort(L, L+N, cmp);
        for(i=0;i<N;i++)
            P[stp][L[i].p] = i > 0 && L[i].nr[0] == L[i-1].nr[0] && L[i].nr[1] == L[i-1].nr[1] ? P[stp][L[i-1].p] : i;
    }

    Sol = -1;

    for(int i=0;i<N-K;i++)
        Sol = max(Sol,lcp(L[i].p,L[i+K-1].p));

    fout << Sol << "\n";
}