#include <bits/stdc++.h>
using namespace std;
int n, k;
int mi = 0, ma = 25;
bool exist[26];
char str[100005];
inline void init() {
  scanf("%d%d", &n, &k);
  memset(exist, false, sizeof(exist));
  scanf("%s", str + 1);
}
inline void solve() {
  for (int i = 1; i <= n; i++) exist[str[i] - 'a'] = true;
  for (mi = 0; !exist[mi]; mi++)
    ;
  for (ma = 25; !exist[ma]; ma--)
    ;
  if (n < k) {
    printf("%s", str + 1);
    for (int i = n + 1; i <= k; i++) putchar(mi + 'a');
    return;
  }
  int p;
  for (p = k; p && str[p] == ma + 'a'; p--)
    ;
  str[p]++;
  while (!exist[str[p] - 'a']) str[p]++;
  for (p = p + 1; p <= k; p++) str[p] = mi + 'a';
  str[k + 1] = 0;
  puts(str + 1);
}
int main() {
  init();
  solve();
  return 0;
}
