#include <iostream>
#include <bits/stdc++.h>
using namespace std;
ifstream fin("substr.in");
ofstream fout("substr.out");
int N,K,i,sol;
char cuv[1<<14],*p[1<<14];
bool cmp(char* x,char* y)
{
    return strcmp(x,y)<0;
}
int main()
{
    fin >> N >> K >> cuv;
    for(i=0;i<N;++i) p[i]=cuv+i;
    sort(p,p+N,cmp);
    for(i=0;i<=N-K;++i)
    {
        int nr=0;
        char *a=p[i],*b=p[i+K-1];
        while(*a==*b and *a and *b) ++nr,++a,++b;
        sol=max(sol,nr);
    }
    fout << sol;
    return 0;
}