import java.util.*;

public class seN {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        Random rand = new Random();
        StringBuilder sb = new StringBuilder();

        char s =sc.next().charAt(0);
        char e = sc.next().charAt(0);

        int N = sc.nextInt();

        switch (N) {
            case 1:
                if(s == e){
                    System.out.println(s);
                }else{
                    System.out.println(-1);
                }   
                break;
            case 2:
                System.out.println(s + e);
                break;
            default:
                sb.append(s);
                char c;
                for(int i = 0; i < N-2; i++){
                    c = (char)('a'+ rand.nextInt(26));
                    sb.append(c);
                }
                sb.append(e);

                System.out.println(sb.toString());
                break;
        }

        sc.close();
        
    }
}