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.util.Scanner;


public class PenguingStrings {
	private static Scanner in = new Scanner(System.in);
	public static void main(String[] args) {
		int n = in.nextInt();
		int k = in.nextInt();
		System.out.println(answer(n,k));
	}
	private static String answer(int n, int k) {
		if (k > n) return "-1";
		if (k == 1) {
			if (n == 1) {
				return "a";
			} else {
				return "-1";
			}
		}
		StringBuilder ret = new StringBuilder();
		int leftover = k - 2;
		char[] toggle = "ab".toCharArray();
		for (int i = 0; i < n - leftover; i++) {
			ret.append(toggle[i%2]);
		}
		String abc = "cdefghijklmnopqrstuvwxyz";
		ret.append(abc.substring(0, leftover)); //cdef....
		return ret.toString();
	}
}
