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.*;
import java.io.*;

public class cf288A {
	static InputReader in = new InputReader();
	static String str;
	static String[] arr;

	public static void main(String[] args) throws Exception {
		 str = in.readLine();
		 int n = Integer.parseInt(str.substring(0, str.indexOf(" ")));
		 int k = Integer.parseInt(str.substring(str.indexOf(" ") + 1));
		 if(n < k) {
		 	System.out.println(-1);
		 	return;
		 }
		 if(n == 1) {
		 	System.out.println("a");
		 	return;
		 }
		 if(k == 1) {
		 	System.out.println(-1);
		 	return;
		 }
		 StringBuilder sb = new StringBuilder("");
		 for(int i = 0; i < n - k + 2; i++) {
		 	sb.append(i % 2 == 0 ? "a" : "b");
		 }
		 for(int i = 0; i < k - 2; i++) {
		 	sb.append((char) (97 + 2 + i));
		 }
		 System.out.println(sb.toString());
	}
	
	static class InputReader {
		BufferedReader br;

		public InputReader() {
			try {
				br = new BufferedReader(new FileReader("cf288A.in"));
			} catch(Exception e) {
				br = new BufferedReader(new InputStreamReader(System.in));
			}
		}

		public String readLine() throws Exception {
			return br.readLine();
		}
	}
}