#include<iostream>
#include<unordered_map>
#include <string>
#include <map>

int main()
{
    std::map<std::string, int> sub_words;
    int line_len, word_len;
    std::cin >> line_len >> word_len;

    std::cin.ignore();
    std::string line;
    std::getline(std::cin, line);

    for(int i=0;i<=line_len - word_len ;i++)
    {
        std::string key = line.substr(i, word_len);

        if(sub_words.find(key) == sub_words.end())
        {
            //not found
            sub_words[key] = 1;
        }
        else
        {
            sub_words[key]++;
        }
    }

    //TODO take only keys with value of word_len
    //TODO find value from behin of list

    int max_number = 0;
    std::string max_key;
    std::map<std::string, int>::reverse_iterator it = sub_words.rbegin();

    max_key = it->first;
    max_number = it->second;
    for(;it != sub_words.rend();++it)
    {
        if(it->second > max_number)
        {
            max_number = it->second;
            max_key = it->first;
        }

    }

    std::cout << max_key<< std::endl;

    return 0;
}