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.*;

public class C {
	static String solve(int n, int k){
		if (n < k){
			return "-1"; 
		}
		if (k == 1){
			if (n == 1){
				return "a";
			}
			return "-1";
		}
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < n - k + 2; ++i){
			final char c = (char)('a' + (i%2));
			sb.append(c);
		}
		for (int i = 2; i <= k - 1; ++i){
			final char c = (char)('a' + i);
			sb.append(c);
		}
		return sb.toString();
	}
	public static void main(String [] args){
		Scanner s = new Scanner(System.in);
		final int n = s.nextInt();
		final int k = s.nextInt();
		System.out.println(solve(n, k));
		s.close();
	}
}
