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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
public class PoloPenguinStrings
{
    public static void main(String[]args) throws IOException
    {
    	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    	StringTokenizer token = new StringTokenizer(br.readLine());boolean flag = false;
    	int n =Integer.parseInt(token.nextToken());
    	int k =Integer.parseInt(token.nextToken());
    	if(k>n||(k==1&&n!=1))flag=true;
    	int letters=n-(k-2);
    	if(flag==false&&n>1)
    	{
    		for(int i=0;i<letters;i++)
    		{
    			if(i%2==0)System.out.print('a');else System.out.print('b');
    		}
    		for(int j=2;j<k;j++)
    		{
    			System.out.print((char)(j+97));
    		}
    	}
    	else if(flag==true)System.out.print("-1");
    	else System.out.print('a');
    	}
    }