#include <bits/stdc++.h>
using namespace std;
int a[30];
int n, k;
string str, s1;
char xl[30], mm;
int main() {
  cin >> n >> k;
  cin >> str;
  for (int i = 0; i < n; i++) {
    int tmp = str[i] - 'a' + 1;
    a[tmp] = 1;
  }
  int cnt = 0;
  for (int i = 0; i <= 29; i++) {
    if (a[i] == 1) {
      char tmp = i - 1 + 'a';
      xl[++cnt] = tmp;
      mm = tmp;
    }
  }
  if (k > n) {
    cout << str;
    for (int i = 1; i <= k - n; i++) cout << xl[1];
    cout << endl;
  } else {
    s1 = str;
    for (int i = k - 1; i >= 0; i--) {
      if (s1[i] == mm)
        s1[i] = xl[1];
      else {
        int tmp = s1[i] - 'a' + 1;
        for (int j = tmp + 1;; j++) {
          if (a[j] == 1) {
            s1[i] = j + 'a' - 1;
            break;
          }
        }
        break;
      }
    }
    for (int i = 0; i < k; i++) cout << s1[i];
    cout << endl;
  }
  return 0;
}
