int main() {
    unordered_map<string, int> table;

    int n, k;
    cin >> n >> k;

    string mostCommon;
    int count = 0;
    string temp;

    cin.get();
    char* input = new char[n];

    scanf("%[^\n]s", input);
    for(int i = 0; i <= n - k; i++){
        char* temp = new char[k + 1];
        strncpy(temp, &input[i], k);
        temp[k] = '\0';
        //cout << temp << endl;

        table[temp]++;
        if(table[temp] >= count){
            count = table[temp];
            mostCommon = temp;
        }
    }
    cout << mostCommon;

    return 0;
}