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 CodeforcesRound177C {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner  kde = new Scanner (System.in );
		int n = kde .nextInt();
		int k = kde .nextInt();
		StringBuffer str= new StringBuffer();
		char a= 97;
		if (n<k)
		{
			System.out.println(-1);
			return;
			
		}
		
		if ((k==1)&&(n>k))
		{
			System.out.println(-1);
			return;
			
		}
		
		
		
		if ((k==1)&&(n==1))
		{
			for(int  i=0; i<(n); i++)
			{
				
				str.append(a);
				
			}
			

			System.out.println(str);	
			return;
		}
		
		
		
		
		
		
		for(int  i=0; i<(n-(k-2)); i++)
		{
			
			str.append(a);
			if(i%2==0)
			{
				a++;
			}
			else
			{
				a--;	
			}	
		}
		 a= 99;
		for(int  i=0; i<(k-2); i++)
		{
			
			str.append(a);
			a++;
		}
		
		System.out.println(str);	
		
		
		
		
	}

}
