#include <bits/stdc++.h>

using namespace std;

ifstream fin("substr.in");
ofstream fout("substr.out");

int n, k, ans;
char *s[30000], c[3000000];

bool cmp(char *x, char *y)
{
    return strcmp(x, y) < 0;
}
int main()
{
    fin >> n >> k;
    fin >> c;
    for(int i = 0; i < n; i++)
        s[i] = c+i;

    sort(s, s+n, cmp);

    for(int i = 0; i < n-k; i++)
    {
        int cnt = 0;
        char *x, *y;
        x = s[i];
        y = s[i+k-1];
        while(*x == *y && *x && *y) cnt++, x++, y++;
        ans = max(ans, cnt);
    }
    fout << ans;
    return 0;
}