#include <fstream>
#include <algorithm>
#include <cstring>
#define NMAX 17000

using namespace std;

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

char s[NMAX], *v[NMAX];

bool comp(char *a, char *b) {
	return strcmp(a,b)<0;
}

int main(){
	int n,k,i,len,res=0;

	fin>>n>>k;
	fin.get();
	fin>>(s+1);


	for(i=1;i<=n;++i) v[i]=s+i;

	sort(v+1,v+n+1,comp);

	for(i=k;i<=n;++i) {
		char *x=v[i-k+1],*y=v[i];
		len=0;

		while(x[len] && y[len] && x[len]==y[len]) ++len;

		res=max(res,len);
	}

	fout<<res;

	return 0;
}