#include <bits/stdc++.h>
const long long int MOD = 1000000007ll;
const int mxsz = 200005;
using namespace std;
double dist(double x1, double y1, double x2, double y2) {
  return (double)sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
}
int main() {
  std::ios_base::sync_with_stdio(false);
  int n, k;
  cin >> n >> k;
  string st;
  cin >> st;
  int ar[26] = {};
  int mn = INT_MAX;
  int mx = 0;
  for (int i = 0; i < n; i++) {
    ar[st[i] - 'a'] = 1;
    mn = min(mn, st[i] - 'a');
    mx = max(mx, st[i] - 'a');
  }
  if (k > n) {
    cout << st;
    k -= n;
    char ch = mn + 'a';
    while (k--) {
      cout << ch;
    }
  } else {
    int i = k - 1;
    char ch = mx + 'a';
    while (1) {
      if (st[i] == ch) {
        i--;
      } else {
        break;
      }
    }
    for (int j = 0; j < i; j++) {
      cout << st[j];
    }
    for (int j = st[i] - 'a' + 1; j < 26; j++) {
      if (ar[j] > 0) {
        char ch = j + 'a';
        cout << ch;
        break;
      }
    }
    ch = mn + 'a';
    for (int j = i + 1; j < k; j++) {
      cout << ch;
    }
  }
  return 0;
}
