#include <bits/stdc++.h>
using namespace std;
void TxtRead() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
}
const int N = 1e5 + 5;
int n, k;
char s[N];
bool presnt[26];
vector<int> x;
map<char, int> mpx;
int main() {
  cin >> n >> k;
  for (int i = 0; i < n; i++) {
    cin >> s[i];
    presnt[s[i] - 'a'] = true;
  }
  for (int i = 0; i < 26; i++) {
    if (presnt[i]) {
      x.push_back(i);
      mpx[char('a' + i)] = x.size() - 1;
    }
  }
  if (k > n) {
    for (int i = 0; i < n; i++) cout << s[i];
    for (int i = 0; i < k - n; i++) cout << char('a' + x[0]);
  } else {
    int strt = 0, i = n - 1;
    while (mpx[s[i]] == x.size() - 1 || i > k - 1) {
      i--;
    }
    strt = i, i = 0;
    while (i != strt) cout << s[i], i++;
    cout << char('a' + x[mpx[s[strt]] + 1]);
    for (i = strt + 1; i < k; i++) cout << char(x[0] + 'a');
  }
}
