#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  long long n, m = 0, i, j = 1, l = 0, r = 0, x = 0, u = 0, y = 1, k = 0,
               ans = 4000000000000000007;
  cin >> n >> k;
  string s;
  cin >> s;
  set<char> p;
  for (i = (0); i <= (n - 1); i++) l = max(l, (long long)s[i]), p.insert(s[i]);
  if (k <= n) {
    char a[k];
    for (i = (k - 1); i >= (1); i--) {
      if (s[i] < l) break;
    }
    for (auto e : p) {
      if (e > s[i]) {
        a[i] = e;
        break;
      }
    }
    for (j = (0); j <= (i - 1); j++) {
      a[j] = s[j];
    }
    for (j = (i + 1); j <= (k - 1); j++) a[j] = *p.begin();
    for (i = (0); i <= (k - 1); i++) cout << a[i];
    return 0;
  } else {
    char a[k];
    for (i = (0); i <= (n - 1); i++) a[i] = s[i];
    for (j = (n); j <= (k - 1); j++) a[j] = *(p.begin());
    for (i = (0); i <= (k - 1); i++) cout << a[i];
    return 0;
  }
}
