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;

public class Main{

	
	public static void main(String[] args) throws IOException {
		BufferedReader t = new BufferedReader(new InputStreamReader(System.in));
		String a[] = t.readLine().split(" ");
		int x = Integer.parseInt(a[0]);
		int y = Integer.parseInt(a[1]);
		if(x==1 && y==1){
			System.out.println("a");
		}else{
		if(x<y || y==1 || y>26){
			System.out.println("-1");
		}else{
			for(int i=0;i<x-y+2;i++){
				if(i%2==0){
					System.out.print("a");
				}else{
					System.out.print("b");
				}
			}
			for(int j=0;j<y-2;j++){
				System.out.print((char) (99+j));
			}
		}
		}
	}

}
