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;

/**
 *
 * @author Do Quoc bao
 */
public class ProblemC {
    public static void main(String[] args) throws java.lang.Exception {
	Scanner in = new Scanner(System.in);
	int n=in.nextInt(),m=in.nextInt(),i;
	if (m>n|(n>1&m==1)) System.out.println(-1);
	else {
	    StringBuilder kq = new StringBuilder();
	    for (i=0;i<n;++i) kq.append(" ");
	    char going=(char)('a'+m-1);
	    for (i=n-1;i>=(n-m+2);--i) {
		kq.setCharAt(i, going);
		--going;
	    }
	    for (i=0;i<Math.min((n-m+2),n);++i) {
		if (i%2==0) going='a'; else going='b';
		kq.setCharAt(i, going);
	    }
	    System.out.println(kq.toString());
	}
    }
}
