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 Main
{
	public static void main (String [] args)
	{
		Scanner scan = new Scanner (System.in);
		int n=scan.nextInt();
		int k=scan.nextInt();
		String alpha = "abcdefghijklmnopqrstuvwxyz  ";
		if ((k<2 && n>1)|| k>n)
			System.out.println(-1);
		else if (k==1 && n==1)
			System.out.println("a");
		else
		{
			int d=(n-k+2)/ 2 ;
			for(int x=0;x<d;x++)
				System.out.print("ab");
			if ((n-k) % 2 == 1)
				System.out.print("a");
			System.out.print(alpha.substring(2,k));
		}

	}
}