#include <bits/stdc++.h>
using namespace std;
map<int, int> m;
pair<bool, char> check(int a, string s) {
  int flag = 0;
  char x = s[a], e;
  for (long long int i = 1 + (x - 'a'); i < 26; i++) {
    if (m.count(i)) {
      return {true, char(i + 'a')};
    }
  }
  return {false, 'f'};
}
int main() {
  ios_base ::sync_with_stdio(false), cin.tie(NULL), cout.tie(0);
  int a, b;
  cin >> a >> b;
  string s;
  cin >> s;
  for (long long int i = 0; i < s.size(); i++) {
    m[s[i] - 'a']++;
  }
  if (a < b) {
    cout << s;
    sort(s.begin(), s.end());
    for (long long int i = 0; i < b - a; i++) cout << s[0];
  } else {
    int k = 0;
    char e;
    for (long long int i = b - 1; i >= 0; i--) {
      pair<bool, char> p = check(i, s);
      if (p.first) {
        e = p.second;
        k = i;
        break;
      }
    }
    for (long long int i = 0; i < k; i++) cout << s[i];
    cout << e;
    sort(s.begin(), s.end());
    if (b - k - 1 > 0) {
      for (long long int i = 0; i < b - k - 1; i++) cout << s[0];
    }
  }
  return 0;
}
