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
/**
 * @(#)qc.java
 *
 *
 * @author 
 * @version 1.00 2013/4/3
 */

import java.util.*;

public class Main{
	
	public static void main (String[] args) {
		
		Scanner cin=new Scanner(System.in);
		
		int n,k,i,j,f=1;
		n=cin.nextInt();
		k=cin.nextInt();
		if(n==1 && k==1)
		{
			System.out.println("a"); f=0;
		}
		else if(n<k|| k==1){
			System.out.println("-1"); f=0;
		}
		if(f==1)
		{
		
		int nd= k-2;
		int c= n-nd;
		
		for(i=1;i<=c;i++)
		{
			if(i%2==1)System.out.print("a");
			else System.out.print("b");
		}
		for(j=0;j<nd;j++)
		{
			
			System.out.printf("%c",j+99);
		}
		System.out.println("");
		}
}
}