#include <fstream>
#include <cstring>

using namespace std;

const int NLIT = 'z' - 'a' + 1;
const int NCIF = 10;
const int BASE = NLIT + NLIT + NCIF;
const int MOD = 1000003;
const int NMAX = 1 << 14;

int N, K, cnt[MOD], poz[MOD];
char S[NMAX];

int normal(char x) {
	if ('a' <= x && x <= 'z')
		return x - 'a';
	if ('A' <= x && x <= 'Z')
		return NLIT + x - 'A';
	return NLIT + NLIT + x - '0';
}

bool occurence(int l) {
	int hash = 0, base = 1;

	for (int i = 0; i < l; ++i)
		base = (base * BASE) % MOD;

	for (int i = 0; i < N; ++i) {
		hash = (normal(S[i]) + hash * BASE) % MOD;
		if (i >= l) {
			hash -= (normal(S[i-l]) * base) % MOD;
			if (hash < 0) hash += MOD;
		}
		if (i + 1 >= l) {
			if (poz[hash] == l) {
				if (++cnt[hash] >= K)
					return true;
			} else {
				poz[hash] = l;
				if ((cnt[hash] = 1) >= K)
					return true;
			}	
		}
	}
	return false;
}

int main(void) {
	ifstream fin("substr.in");
	ofstream fout("substr.out");

	fin >> N >> K >> S;

	int step, length;

	for (step = 1; (step << 1) < N; step <<= 1);
	for (length = 0; step; step >>= 1)
		if (occurence(length + step))
			length += step;

	fout << length << '\n';

	fin.close();
	fout.close();
}