#include <iostream>
#include <tr1/unordered_map>
#include <string>
#include <fstream>

using namespace std;

typedef unsigned long long ull;

const int MAX_N = 16384 + 1;

int n, k;

tr1::unordered_map<ull, int> H;
char s[MAX_N];

const int PWR = 137;
ull p[MAX_N];



bool good(const int len) {
    H.clear();
    ull hash = 0;

    int ret = 1;

    for(int i = 0 ; i < len ; ++i) {
        hash = hash * PWR + s[i];
    }

    H[hash]++;

    for(int i = len ; i < n ; ++i) {
        hash -= s[i - len] * p[len - 1];
        hash = hash * PWR + s[i];
        H[hash]++;
        if(H[hash] > ret)
            ret = H[hash];
    }

    return ret >= k;
}

int bsearch() {
    int i, step;
    for(step = 1 ; step < n ; step <<= 1);

    for(i = 0 ; step ; step >>= 1) {
        if(i + step <= n && good(i + step))
            i += step;
    }
    return i;
}

int main() {
    ifstream in("substr.in");
    in >> n >> k;
    in >> s;
    in.close();

    p[0] = 1;
    for(int i = 1 ; i <= n ; ++i)
        p[i] = p[i-1] * PWR;

    ofstream out("substr.out");
    out << bsearch() << "\n";
    out.close();
}