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.StringTokenizer;


public class PoloAndStrings {
	public static void main(String[]args) throws IOException{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st = new StringTokenizer(br.readLine());
		String s = "abcdefghijklmnopqrstuvwxyz";
		StringBuilder ab = new StringBuilder();
	    for(int i=0;i<500000;i++){
	    	ab.append("ab");}
		int n = Integer.parseInt(st.nextToken());
		int k = Integer.parseInt(st.nextToken());
		if(k>n)System.out.println(-1);
		else if(k==1 && n!=1) System.out.println(-1);
		else {
			StringBuilder sb = new StringBuilder();

			
			if(n>1){
			sb.append(ab.substring(0,n-(k-2)));
			sb.append(s.substring(2,k));}
			else {
				sb.append("a") ;
			}
			System.out.println(sb);}
		}
		
		
	}


