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.*;
import java.util.*;

public class Main{
	public static void main(String[] args) throws IOException {
		BufferedReader Bf = new BufferedReader(new InputStreamReader(System.in));
		String input = Bf.readLine();
		String out = "";

		StringTokenizer str = new StringTokenizer(input, " ");
		int i = 2;
		int n = Integer.parseInt(str.nextToken());
		int k = Integer.parseInt(str.nextToken());
		String e = "";
		String q = "";
		if (n == 1 && k == 1) {
			System.out.println("a");
		} else if (n >= k && !(k == 1 && n > 1)) {
			while (out.length() < n - (k - 2)) {
				if (e.length() == 0) {
					e = "ab";
					q = e;
				} else if (e.length() < n) {

					e += e;
			
				} else {
					int w = n-(k-2);
					out = e.substring(0,w);
					
			}}
			int z = 98;
			while (out.length() < n) {
				out += (char) (++z);
			}
			System.out.println(out);
		} else {
			System.out.println("-1");
		}
	
	}
}
