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 NumberFormatException, IOException {
	BufferedReader x=new BufferedReader(new InputStreamReader(System.in));
    String s=x.readLine();
    String[]a=s.split(" ");
    int n= Integer.parseInt(a[0]);
    int k=Integer.parseInt(a[1]);
    int diff=n-k;
    String letters="abcdefghijklmnopqrstuvwxyz";
    int d=2;
    long f=Math.round((double)diff/2);
    if(n==1&k==1)
    	System.out.println("a");
    else if(k==n){
    	for(int i=0;i<n;i++){
    		System.out.print(letters.charAt(i));
    	}
    	
    }
    else if(k<n){
    	if(k==1){
    		System.out.println("-1");return;}
    	else   if(n%2==0&&k%2==0){
    		f=f+1;
    	}
    	else if(n%2!=0&&k%2!=0)
    		f=f+1;
    	
    		
    	for(long j=0;j<f;j++){
    		System.out.print("ab");  
    	  }
    	
    	
    	
    	
    	if(n-f*2>k-2){
    		System.out.print('a');
    	}
    	for(int t=0;t<k-2;t++){
    		System.out.print(letters.charAt(d));
    		d++;	
    	}
    	}
    else {
    	System.out.println("-1");
    	  	
    	
    }
  
    
    
    
    
}}