#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int x = 0, f = 1;
  char ch;
  do {
    ch = getchar();
    if (ch == '-') f = -1;
  } while (ch < '0' || ch > '9');
  do {
    x = x * 10 + ch - '0';
    ch = getchar();
  } while (ch >= '0' && ch <= '9');
  return x * f;
}
const int N = 100010;
char ch[N];
bool appear[26];
int nxt[26];
inline char getnext(char x) {
  x -= 'a';
  if (nxt[x] != -1) return nxt[x] + 'a';
  nxt[x] = (x + 1) % 26;
  while (!appear[nxt[x]]) nxt[x] = (nxt[x] + 1) % 26;
  return nxt[x] + 'a';
}
int main() {
  int n = read(), k = read();
  scanf("%s", ch + 1);
  for (int i = 0; i < 26; ++i) nxt[i] = -1;
  for (int i = 1; i <= n; ++i) {
    appear[ch[i] - 'a'] = 1;
  }
  int flag = 1;
  ch[k + 1] = 0;
  if (k > n) {
    int mn = 0;
    while (!appear[mn]) ++mn;
    for (int i = n + 1; i <= k; ++i) ch[i] = mn + 'a';
    printf("%s", ch + 1);
    return 0;
  }
  for (int i = k; i >= 1; --i) {
    if (!flag) break;
    char v = getnext(ch[i]);
    if (v > ch[i]) flag = 0;
    ch[i] = v;
  }
  printf("%s", ch + 1);
  return 0;
}
