#include <bits/stdc++.h>
using namespace std;
long long a[2000010], d[2000010];
long long b[200010], f[1010][1010];
long long mn = 4e18, mx = -4e18, ans, sm = 1, cnt, n, m, cur;
bool ok, u[1000010], uu[310][310];
char c[510][510], cc[100010];
int dx[4] = {0, 1, 0, -1}, dy[4] = {-1, 0, 1, 0};
vector<long long> g[900010];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long k;
  cin >> n >> k;
  string s, t = "";
  cin >> s;
  vector<char> v;
  for (int i = 0; i < s.size(); i++) {
    ok = false;
    for (int j = 0; j < v.size(); j++) {
      if (v[j] == s[i]) {
        ok = true;
        break;
      }
    }
    if (!ok) v.push_back(s[i]);
  }
  sort(v.begin(), v.end());
  if (k > s.size()) {
    cout << s;
    for (int i = s.size(); i < k; i++) {
      cout << v[0];
    }
    return 0;
  } else if (k < s.size()) {
    t = s.substr(0, k);
    ok = false;
    for (int i = k - 1; i >= 0; i--) {
      for (int j = 0; j < v.size(); j++) {
        if (v[j] > t[i]) {
          t[i] = v[j];
          ok = true;
          break;
        }
      }
      if (ok) {
        for (int ii = i + 1; ii < k; ii++) {
          t[ii] = v[0];
        }
        cout << t;
        return 0;
      }
    }
  } else {
    t = s;
    ok = false;
    for (int i = k - 1; i >= 0; i--) {
      for (int j = 0; j < v.size(); j++) {
        if (v[j] > t[i]) {
          t[i] = v[j];
          ok = true;
          break;
        }
      }
      if (ok) {
        for (int ii = i + 1; ii < k; ii++) {
          t[ii] = v[0];
        }
        cout << t;
        return 0;
      }
    }
  }
  return 0;
}
