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.Scanner;

public class C {
    public static void main (String args[]){
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int k = in.nextInt();
        if(k>n){
            System.out.println(-1);
            return;
        }
        if(k==1 && n > 1){
            System.out.println(-1);
            return ;
        }
        StringBuffer sb = new StringBuffer();
        int t = n - k + 2;
        char c = 'c' ;
        for(int i = 0 ; i < n ;i++){
            if(i < t){
                if(i%2==0)
                    sb.append("a");
                else
                    sb.append("b");
            }
            else {
                sb.append(c);
                c++;
            }
        }
        System.out.println(sb.toString());
    }

}
