#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,sse3,sse4,popcnt,abm,mmx")
using namespace std;
const double eps = 0.000001;
const long double pi = acos(-1);
const int maxn = 1e7 + 9;
const int mod = 1e9 + 7;
const long long MOD = 1e18 + 9;
const long long INF = 1e18 + 123;
const int inf = 2e9 + 11;
const int mxn = 1e6 + 9;
const int N = 3e5 + 123;
const int PRI = 555557;
const int pri = 997;
int tests = 1;
int n, m, k;
string s;
int cnt[31];
int cur, res;
int ans;
set<int> q;
inline void Solve() {
  cin >> n >> k;
  cin >> s;
  for (int i = 0; i < s.size(); i++) cnt[s[i] - 'a'] = 1;
  if (k > n) {
    cout << s;
    for (int j = n + 1; j <= k; j++)
      for (int i = 0; i < 26; i++) {
        if (cnt[i]) {
          cout << char(i + 'a');
          break;
        }
      }
    exit(0);
  }
  n = k;
  for (int i = n - 1; i >= 0; i--) {
    for (int j = s[i] - 'a' + 1; j < 26; j++) {
      if (cnt[j]) {
        s[i] = j + 'a';
        cur = i;
        break;
      }
    }
    if (cur) break;
  }
  for (int i = 0; i <= cur; i++) cout << s[i];
  cur++;
  for (; cur < n; cur++) {
    for (int i = 0; i < 26; i++) {
      if (cnt[i]) {
        cout << char(i + 'a');
        break;
      }
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0);
  while (tests--) {
    Solve();
  }
  return 0;
}
