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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;



public class Strings {

	/**
	 * @param args
	 * @throws IOException 
	 */
	public static void main(String[] args) throws IOException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

		String s2 = "";
		String[] lineOne = null;
		if ((s2 = in.readLine()) != null)
			lineOne = s2.split("\\s+");
		
		int n = Integer.parseInt(lineOne[0]);
		int k = Integer.parseInt(lineOne[1]);

		in.close();
				
		if (n<k) {
			System.out.println(-1);
			return;
		}
		
		if (k == 1 && n > 1) {
			System.out.println(-1);
			return;
		}
			
		
		StringBuilder s = new StringBuilder(k);
		for (int i = 0; i < k; i++) {
			s.append(String.valueOf((char)(i+97)));
		}
		
		StringBuilder tmpS = new StringBuilder(n-k);
		for (int i = 0; i < n-k; i++) {
			tmpS.append(String.valueOf((char)((i%2)+97)));
		}
		
		if (s.length() >= 2)
			System.out.println(s.substring(0, 2)+tmpS+s.substring(2, s.length()));
		else
			System.out.println(s);

		
	}

}
