#include <bits/stdc++.h>
using namespace std;
int n, kk, k, used[1000000], c, thien, tmp;
char tt, a[1000000], s[1000001];
int main() {
  cin >> n >> kk;
  for (int i = 1; i <= n; i++) cin >> s[i];
  for (int i = 1; i <= n; i++)
    if (used[s[i]] == 0) {
      used[s[i]] = 1;
    }
  for (char i = 'a'; i <= 'z'; i++) {
    if (used[i] != 0) {
      c++;
      a[c] = i;
      if (c == 1) tt = i;
    }
  }
  thien = -1;
  if (n < kk) {
    for (int i = 1; i <= kk; i++)
      if (i <= n)
        cout << s[i];
      else
        cout << tt;
  } else {
    k = kk;
    for (int i = k; i >= 1; i--) {
      for (int j = 1; j <= c; j++) {
        if (a[j] - s[i] > 0) {
          thien = i;
          tmp = j;
          break;
        }
      }
      if (thien != -1) break;
    }
    for (int i = 1; i < thien; i++) cout << s[i];
    cout << a[tmp];
    for (int i = thien + 1; i <= k; i++) cout << tt;
  }
}
