#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n, k;
  cin >> n >> k;
  set<char> s;
  string a1;
  cin >> a1;
  for (char i : a1) s.insert(i);
  int len = s.size();
  vector<char> v;
  set<char>::iterator it;
  for (it = s.begin(); it != s.end(); ++it) {
    v.push_back(*it);
  }
  map<char, char> make_pair;
  for (int i = 0; i < len; i++) {
    make_pair[v[i]] = v[(i + 1) % len];
  }
  int toNote = v[len - 1];
  int startpos = k - 1;
  if (k <= n) {
    string a2(a1, 0, k);
    for (int i = 0; i <= startpos; i++) {
      a2[i] = a1[i];
    }
    for (int i = startpos; i >= 0; i--) {
      if (a1 >= a2 && a2[i] != toNote) {
        a2[i] = make_pair[a2[i]];
        break;
      } else if (a1 >= a2 && a2[i] == toNote) {
        a2[i] = make_pair[a2[i]];
        a2[i - 1] = make_pair[a2[i - 1]];
        i--;
      } else {
        break;
      }
    }
    cout << a2 << '\n';
  } else {
    string a2(a1, 0, n);
    cout << a2;
    for (int i = 0; i < k - n; i++) {
      cout << v[0];
    }
    cout << '\n';
  }
  return 0;
}
