#include<iostream>
#include <string>
#include<map>
using namespace std;
int main() {
    std::map<std::string, int>m;
    int sizeOfText;
    int sizeOfWord;
    std::string text;
    std::cin >> sizeOfText >> sizeOfWord;
    std::cin >> text;
    std::string curr="";
    for (size_t i = 0; i <= sizeOfText- sizeOfWord; i++) {
        curr = text.substr(i,sizeOfWord);
        m[curr]++;
    }
    int max = 0;
    string maxS;
    for (auto el : m) {
        if (el.second > max) {
            max = el.second;
            maxS = el.first;
        }
    }
    cout << maxS;
    return 0;
}