#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
stringstream output;
inline void eOP();
inline void solve() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  bool* letters = new bool[26]();
  for (int i = 0; i < n; ++i) {
    letters[s[i] - 'a'] = true;
  }
  vector<char> decode;
  for (int i = 0; i < 26; ++i) {
    if (letters[i]) {
      decode.push_back(i + 'a');
    }
  }
  unordered_map<char, int> encode;
  for (int i = 0; i < decode.size(); ++i) {
    encode[decode[i]] = i;
  }
  vector<int> t;
  t.reserve(k);
  const size_t L = min(n, k);
  for (int i = 0; i < L; ++i) {
    t.push_back(encode[s[i]]);
  }
  t.resize(k);
  if (k <= n) {
    int i = k - 1;
    ++t[i];
    while (t[i] == decode.size()) {
      t[i] = 0;
      --i;
      ++t[i];
    }
  }
  for (int code : t) {
    output << decode[code];
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  size_t t = 1;
  for (int i = 0; i < t; ++i) {
    solve();
  }
  eOP();
}
inline void eOP() {
  cout << output.str();
  output.str(string());
  exit(0);
}
