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

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

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


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

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

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

    /*for(auto et=sub_words.begin();et!=sub_words.end();++et)
    {
        std::cout << et->first << "=" << et->second << std::endl;
    }*/
    return 0;
}