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 third {

	/**
	 * @param args
	 */
	static String fast_con(int k)
	{
		if(k==1)return "ab";
		if(k%2==0)
		{
			String con = fast_con(k/2);
			return con+con;
		}
		String con = fast_con(k/2);
                return con+con+"ab";
	}
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int k = sc.nextInt();
		if(k>n)System.out.println(-1);
		else if(k==1 && n>1)System.out.println(-1);
		else if(n==1)System.out.println("a");
		else
		{
			int j = (n-k+2)/2;
			String response = fast_con(j);
			if((n-k+2)%2==1)response+="a";
			String[] alphabet = { "c", "d", "e", "f", "g",
					"h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t",
					"u", "v", "w", "x", "y", "z" };
			for(int i=0;i<k-2;i++)
			{
				response+=alphabet[i];
			}
			System.out.println(response);
		}
	}

}
