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 A177 {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner obj=new Scanner(System.in);
		int n=obj.nextInt();
		int k=obj.nextInt();
		StringBuilder sb=new StringBuilder();
		StringBuilder sb1=new StringBuilder();
		if(k>n||(k==1&&n>1))
			System.out.println("-1");
		else
		{
			char c='c';
			if(k>2)
			{
				k=k-2;
				for(int i=k;i>=1;i--)
				{
					sb.append(c);
					c++;
					n--;
				}
			}
			char ch='a';
			for(int i=0;i<n;i++)
			{
				sb1.append(ch);
				if(ch=='a')
					ch='b';
				else if(ch=='b')
					ch='a';
			}
		}
		System.out.print(sb1);
		System.out.print(sb);

	}

}
