#include <bits/stdc++.h>
#pragma GCC optimize(2)
using namespace std;
const int INF = 1e9 + 7;
const double eps = 1e-6;
const int MAXN = 1e6 + 7;
int n, k;
char s[MAXN];
bool vis[30];
int main() {
  int l = INT_MAX, r = INT_MIN;
  cin >> n >> k;
  cin >> s;
  for (int i = 0; s[i]; i++) {
    vis[s[i] - 'a'] = 1;
    l = min(l, s[i] - 'a');
    r = max(r, s[i] - 'a');
  }
  if (n < k) {
    cout << s;
    for (int i = n + 1; i <= k; i++) cout << char(l + 'a');
    cout << endl;
  } else {
    int i, j;
    for (i = k - 1; ~i; i--)
      if (r + 'a' > s[i]) {
        for (int t = 0; t < i; t++) cout << s[t];
        for (j = s[i] - 'a' + 1; j <= r; j++)
          if (vis[j]) {
            cout << char(j + 'a');
            break;
          }
        for (j = i + 1; j < k; j++) cout << char(l + 'a');
        cout << endl;
        break;
      }
  }
  return 0;
}
