#include <bits/stdc++.h>

using namespace std;

int main()
{
    string s,res;
    int n,k;
    char min_char;
    cin >> n >> k >> s;
    res = s;
    for (int j = 0; j <= 25; j++)
        if (s.find('a' + j) != -1)
        {
            min_char = 'a' + j;
            break;
        }


    if (k > n)
    {
        while (res.size() < k)
            res += min_char;
        cout << res << endl;
        return 0;
    }

    while (res.size() > k)
        res.pop_back();

    for (int i = res.size() - 1; i >= 0; i--)
    {
        bool f = true;
        for (int j = 0; j <= 25; j++)
            if (s.find('a' + j) != -1 && 'a' + j > s[i])
            {
                res[i] = 'a' + j;
                f = false;
                break;
            }

        if (!f)
            break;
        else
            res[i] = min_char;
    }
    cout << res << endl;
}