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.BufferedOutputStream;
import java.util.Scanner;


public class Polo {
    public static void main(String[] args)
    {
        int n, k;
        Scanner in = new Scanner(System.in);
        n = in.nextInt();
        k = in.nextInt();
        StringBuilder sb = new StringBuilder();
        if(k == 1)
        {
            if(n!=1)
            {
                System.out.println("-1");
                System.exit(0);
            }
            else
            {
                System.out.println("a");
                System.exit(0);
            }
        }
        if(k > n)
        {
            System.out.println("-1");
            System.exit(0);
        }
        while(n > k-2)
        {
            if(n-1>k-2)
            {
                sb.append("ab");
                //System.out.print("ab");
                n-=2;
            }
            else
            {
                sb.append("a");
                //System.out.print("a");
                n--;
            }
        }
        char c = 'c';
        while(n>0)
        {
            sb.append(c);
            c++;
            n--;
        }
        System.out.println(sb);
    }
}
