#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  vector<long long> v(26);
  for (long long i = 0; i < n; i++) {
    v[s[i] - 'a']++;
  }
  if (n < k) {
    cout << s;
    for (long long i = 0; i < 26; i++) {
      if (v[i]) {
        long long t = k - n;
        while (t--) cout << char('a' + i);
        return 0;
      }
    }
  }
  string ans;
  for (long long i = k - 1; i >= 0; i--) {
    long long p = s[i] - 'a';
    for (long long j = p + 1; j < 26; j++) {
      if (v[j]) {
        for (long long x = 0; x < i; x++) {
          cout << s[x];
        }
        cout << char(j + 'a');
        long long x = k - 1 - i;
        for (long long d = 0; d < 26; d++) {
          if (v[d]) {
            for (long long g = 0; g < x; g++) {
              cout << char('a' + d);
            }
            return 0;
          }
        }
      }
    }
  }
  return 0;
}
