#include <bits/stdc++.h>
using namespace std;
int sum[200005];
int main() {
  ios::sync_with_stdio(0);
  int t = 1;
  while (t--) {
    int n, k;
    cin >> n >> k;
    string s, t = "";
    cin >> s;
    map<char, int> make_pair;
    for (int i = 0; i < n; i++) make_pair[s[i]] = 1;
    for (int i = 0; i < min(k, n); i++) t += s[i];
    char mn;
    for (char i = 'a'; i <= 'z'; i++) {
      if (make_pair[i] == 1) {
        mn = i;
        break;
      }
    }
    if (k > n) {
      for (int i = n; i < k; i++) s += mn;
      cout << s << endl;
      return 0;
    }
    for (int i = k - 1; i >= 0; i--) {
      char ch = s[i];
      bool f = 0;
      for (char j = char(ch + 1); j <= 'z'; j++) {
        if (make_pair[j] == 1) {
          t[i] = j;
          f = 1;
          break;
        }
      }
      if (f == 1)
        break;
      else
        t[i] = mn;
    }
    cout << t << endl;
  }
  return 0;
}
