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.io.*;
import java.util.*;
import java.util.TreeSet;
import java.lang.*;
public class ab{
	public static void main(String args[]){
		Scanner in=new Scanner(System.in);
		int n=in.nextInt(),k=in.nextInt(),i;
		if(n==k&&n==1)
			System.out.println("a");
		else if(n>=k&&k>=2){
			int t=n-k+2;
			StringBuilder sb=new StringBuilder();
			for(i=0;i<t/2;i++){
				sb=sb.append("ab");
			}
			if(t%2!=0)
				sb=sb.append("a");
			for(i=0;i<k-2;i++){
				char c=(char)(99+i);
				sb=sb.append(c);
			}
			String z=sb.toString();
			System.out.println(z);
		}
		else
			System.out.println(-1);
		
	}
}