#include <fstream>
#include <unordered_map>
#include <cstring>
using namespace std;

const int kMaxN = 16390;
const int kMaxLg = 16;

int N, K;
char a[kMaxN];

int lg2[kMaxN];
int indx[kMaxLg][kMaxN];
unordered_map<int, int> id;
int cnt[kMaxN];

void MakeRMQ() {
	for (int i = 2; i < kMaxN; ++i)
		lg2[i] = lg2[i >> 1] + 1;

	for (int i = 1; i <= N; ++i)
		indx[0][i] = a[i];

	for (int i = 1; i < kMaxLg; ++i) {
		memset(cnt, 0, sizeof cnt);
		int id_crt = 0;
		id.clear();

		for (int j = 1; j <= N - (1 << i) + 1; ++j) {
			int crt = indx[i - 1][j] * kMaxN + indx[i - 1][j + (1 << (i - 1))];
			if (!id.count(crt))
				id[crt] = ++id_crt;
			indx[i][j] = id[crt];
		}
	}
}

bool Check(int len) {
	id.clear();
	int lg = lg2[len];
	memset(cnt, 0, sizeof cnt);
	int id_crt = 0;

	for (int i = 1; i <= N - len + 1; ++i) {
		int crt = indx[lg][i] * kMaxN + indx[lg][i + len - (1 << lg)];
		if (!id.count(crt))
			id[crt] = ++id_crt;
		if (++cnt[id[crt]] == K)
			return true;
	}

	return false;
}

int Solve() {
	int ans = 0;
	for (int step = 1 << 14; step; step >>= 1)
		if (Check(ans | step))
			ans |= step;
	return ans;
}

int main() {
	ifstream("substr.in") >> N >> K >> (a + 1);
	MakeRMQ();
	ofstream("substr.out") << Solve() << "\n";
	return 0;
}