#include<algorithm>
#include<bitset>
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<deque>
#include<fstream>
#include<iostream>
#include<map>
#include<queue>
#include<set>
#include<stack>
#include<utility>
#include<vector>

using namespace std;

#define dbg(x) (cout<<#x<<" = "<<(x)<<'\n')
const string problemName = "substr";
const string inputFile = problemName + ".in";
const string outputFile = problemName + ".out";

typedef long long int lld;
typedef pair<int, int> PII;

const int INF = (1LL << 31) - 1;
const lld LINF = (1LL << 62) - 1;
const int NMAX = 100000 + 5;

int N, K, step, length, sol;
char S[NMAX];
int P[4 * NMAX][18];
int T[NMAX];

bool cmp(int A, int B) {
    if(P[A][step] == P[B][step])
        return (P[A + length][step] < P[B + length][step]);
    return (P[A][step] < P[B][step]);
}

void suffix_array() {
    int i, j;

    for(i = 1; i <= N; i++) {
        P[i][0] = S[i] - 'a' + 1;
        T[i] = i;
    }

    for(step = 0; (1 << step) <= N; step++) {
        length = (1 << step);

        sort(T + 1, T + N + 1, cmp);

        for(i = 1; i <= N; i = j) {
            for(j = i; j <= N; j++)
                if(P[T[i]][step] == P[T[j]][step] && P[T[i] + length][step] == P[T[j] + length][step])
                    P[T[j]][step + 1] = i;
                else
                    break;
        }
    }
}

int lcp(int x, int y) {
    int i, ans = 0;

    for(i = step; i >= 0; i--) {
        if(P[x][i] == P[y][i]) {
            ans += (1 << i);
            x += (1 << i);
            y += (1 << i);
        }
    }

    return ans;
}

int main() {
    int Q, i, x, y;

#ifndef ONLINE_JUDGE
    freopen(inputFile.c_str(), "r", stdin);
    freopen(outputFile.c_str(), "w", stdout);
#endif

    scanf("%d%d", &N, &K);
    scanf("%s", S + 1);

    suffix_array();

    for(i = 1; i <= N - K + 1; i++)
        sol = max(sol, lcp(T[i], T[i + K - 1]));

    printf("%d\n", sol);

    return 0;
}