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.*;

public class main
{
	public static void main(String[] args) {
		
		BufferedReader inputStream = new BufferedReader(new InputStreamReader(System.in));
		BufferedWriter outputStream = new BufferedWriter(new OutputStreamWriter(System.out));

		String alphabet = new String("abcdefghijklmnopqrstuvwxyz");
		//String output = "";
		try
		{
			String[] lengthLetters = inputStream.readLine().split(" ");
			int length = Integer.parseInt(lengthLetters[0]);
			int chars = Integer.parseInt(lengthLetters[1]);

			/*
				Careless mistakes!.
			*/
			if(length >= chars && chars > 1)
			{
				for(int i=0;i<length-chars+2;i++)
				{
					if(i%2 == 0)
						outputStream.write("a");
						//output += "a";
					else
						outputStream.write("b");
						//output += "b";
				}

				for(int i = 2;i<chars;i++)
					outputStream.write(alphabet.charAt(i));
					//output += Character.toString(alphabet.charAt(i));


				//System.out.println(output);
			}
			else if(length == chars && chars == 1)
				outputStream.write("a");
			else
				outputStream.write("-1");

			outputStream.flush();
		}
		catch(IOException ioexcept)
		{
			/*
			*/
			System.out.printf("IOEREORR");
		}

	}
}