#include <iostream>
#include <fstream>
#include <cstring>
#include <algorithm>
using namespace std;
ifstream f ("substr.in");
ofstream g ("substr.out");
int n, k, ans;
char *s[32005], c[3000005];

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


    sort(s, s+n, comp);
    int r=0;
    for(int i=0;i<=n-k;++i)
    {
        int nr=0;
        char *x,*y;
        x=s[i];
        y=s[i+k-1];
        while(*x==*y && *x && *y) {
            nr++;
            x++;
            y++;
        }
       r=max(r, nr);
    }
    g<<r;
    return 0;
}