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.util.Arrays;
import java.util.Scanner;

import java.util.Scanner;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Map;
import java.util.Queue;
import java.util.ArrayList;
public class Main {

 
    public static void main(String[] args) 
    {
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        int k = scan.nextInt();
//      long sum = 0;
//      for(int i = 0;i<n;i++)
//      {
//          int a = scan.nextInt();
//          int b = scan.nextInt();
//          sum+=(b-a)+1;
//           
//      }
//      System.out.println(k-sum%k==k?0:k-sum%k);
        StringBuilder sb = new StringBuilder("");
        if(k==1&&n==1)
        {
            System.out.println('a');
            return;
        }
        if(n<k||k==1)
        {
            System.out.print(-1);
            return;
        }

        for(int i = 0;i<=n-k+1;i++)
        {
            if(i+1==(n-k+2))
                sb.append("a");
            else
            {
                sb.append("ab");
                i++;
            }
                
        }
        char g;
        
        if(sb.length()==0)
            g='a';
        else
            if(sb.length()==1)
                g='b';
            else
                g='c';
        

        int c = 0;
        for(int i = 0;i<k-2;i++)
        {
            sb.append(g);
            g++;
        }
        System.out.print(sb);
        
         
        
    }
 
    
}