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.*;
import java.io.*;
public class Polo_the_Penguin_and_Strings
{
	public static void main(String args[]) throws Exception
	{
		BufferedReader f=new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st=new StringTokenizer(f.readLine());
		int length=Integer.parseInt(st.nextToken());
		int distinct=Integer.parseInt(st.nextToken());
		StringBuilder ans=new StringBuilder(length);
		int start=length-distinct+2;
		if(distinct>length)
			System.out.println(-1);
		else
		{
			if(distinct==1)
			{
				if(length==1)
					System.out.println("a");
				else
					System.out.println(-1);
			}
			else
			{
				int temp=0;
				for(int x=0;x<start;x++)
				{
					ans.append((char)(temp+97));
					temp++;
					temp%=2;
				}
				char cur='c';
				for(int x=start;x<length;x++)
				{
					ans.append(cur);
					cur++;
				}
				System.out.println(ans);
			}
		}
	}
}