#include <iostream>
#include <vector>
#include <algorithm>
#include <fstream>

using namespace std;

int main()
{
int n, k;
char f;
cin >> n >> f >> k;
int N = 26;
char slovo[n];
for(int i = 0; i < n; i++) {
    cin >> slovo[i];
}
int letters[N];
for(int i = 0; i < n; i++){
    if(letters[int(slovo[i]) - int('a')] == 0){
        letters[int(slovo[i]) - int('a')] = 1;
    }
}
    int min = 27;
    int max = 0;
    for(int i = 0; i < N; i++) {
        if(min > letters[i] && letters[i] == 1) {
            min = letters[i];
        }
        if(max < letters[i] && letters[i] == 1) {
            max = letters[i];
        }
    }
           int save = k;
    if (k > n) {
        for(int i = 0; i < n; i++){
            cout << slovo[i];
        }
        for(int i = 0; i < k - n; i++){
            cout << char(int('a') + min);
        }
        return 0;
        } else {
            while(slovo[k] == max) {
                slovo[k] = min;
               k--;
            }
            for(int i = int(slovo[k])-int('a') + 1; i < N; i++){
                if(letters[i] == 1){
                slovo[k] = letters[i];
                }
            }
        }
    for(int i = 0; i < save; i++){
        cout<<slovo[i];
    }
    return 0;
}