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.ArrayList;
import java.util.Collections;
import java.util.Scanner;


public class PenguinStrings {
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		
		Scanner s = new Scanner(System.in);
		
		int length = s.nextInt();
		int numLetters = s.nextInt();
		
		String alphabet = "abcdefghijklmnopqrstuvwxyz";
		
		boolean solvable = true;
		StringBuffer answer = new StringBuffer("-1");
		if (length - numLetters < 0)
		{
			solvable = false;
		}
		if (numLetters <= 1)
		{
			if (length == 1)
			{
				answer = new StringBuffer("a");
			}
			solvable = false;
		}
		
		
		if (solvable)
		{
			answer = new StringBuffer("");
			
			// Generate ababa... for start of string
			int start_length = length - numLetters + 2;
			for (int i = 0; i < start_length; i++)
			{
				if (i % 2 == 0)
				{
					answer.append("a");
				}
				else
				{
					answer.append("b");
				}
			}
			
			// Generate string to use remaining letters
			int end_length = length - start_length;
			for (int i = 0; i < end_length; i++)
			{
				answer.append(alphabet.substring(2+i, 2+i+1));
			}
		}
		
		System.out.println(answer.toString());
	}
}
