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 Code176Adiv1 {

    public static void main(String[] args) {
        
        int n,k;
        Scanner scanner = new Scanner(System.in);
        n = scanner.nextInt();
        k = scanner.nextInt();
        if((k>n) || (n!=1 && k==1)){
            System.out.println("-1");
            System.exit(0);
        }
        
        char[] alpha = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
        
        if(n==k && n<=26){
            for(int i=1;i<=n;i++)
                System.out.print(alpha[i-1]);
        }else{
            k-=2;
            for(int i=0;i<n-k;i++) System.out.print(alpha[i%2]);
            for(int i=n-k,m=2;i<n;i++,m++) System.out.print(alpha[m]);
        }
    }

}
