Little penguin Polo adores strings. But most of all he adores strings of length n.

One day he wanted to find a string that meets the following conditions:

  1. The string consists of n lowercase English letters (that is, the string's length equals n), exactly k of these letters are distinct. 
  2. No two neighbouring letters of a string coincide; that is, if we represent a string as s = s1s2... sn, then the following inequality holds, si ≠ si + 1(1 ≤ i < n). 
  3. Among all strings that meet points 1 and 2, the required string is lexicographically smallest. 



Help him find such string or state that such string doesn't exist.

String x = x1x2... xp is lexicographically less than string y = y1y2... yq, if either p < q and x1 = y1, x2 = y2, ... , xp = yp, or there is such number r (r < p, r < q), that x1 = y1, x2 = y2, ... , xr = yr and xr + 1 < yr + 1. The characters of the strings are compared by their ASCII codes.

Input

A single line contains two positive integers n and k (1 ≤ n ≤ 106, 1 ≤ k ≤ 26) — the string's length and the number of distinct letters.

Output

In a single line print the required string. If there isn't such string, print "-1" (without the quotes).

Examples

Input

7 4


Output

ababacd


Input

4 7


Output

-1
import java.io.BufferedInputStream;
import java.util.Scanner;

/**
 * @author Evgeniy Kiselev
 *         Date: 02.04.13
 */
public class C {

    public static void main(String[] arg) {
        final Scanner scanner = new Scanner(new BufferedInputStream(System.in));
        final String[] input = scanner.nextLine().split(" ");
        final int N = Integer.parseInt(input[0]);
        final int K = Integer.parseInt(input[1]);
        if (N < K || (K == 1 && N != 1)) {
            System.out.println("-1");
            return;
        }
        if (K == 1) {
            System.out.println('a');
            return;
        }
        StringBuilder result = new StringBuilder();
        boolean isA = false;
        for (int i = 0; i < N - K + 2; i++) {
            if (!isA) {
                result.append('a');
            } else {
                result.append('b');
            }
            isA = !isA;
        }
        for (int i = 0; i < K - 2; i++) {
            result.append((char)('c' + i));
        }


        System.out.println(result.toString());
    }
}
