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 marte{


private int contor=0;

public static void main (String args[]) throws Exception{

Scanner input=new Scanner(System.in);

HashMap<String, Integer> map= new HashMap<String, Integer>();
HashMap<String, Integer> map2= new HashMap<String, Integer>();
HashMap<Integer, Integer> finish= new HashMap<Integer, Integer>();
HashMap<Integer, Integer> maxime= new HashMap<Integer, Integer>();

	
	int n=input.nextInt();
	int k=input.nextInt();
	
	StringBuilder  str2=new StringBuilder ();
	
	String str="abcdefghijklmnopqrstuvwxyz";
	
//	for(int j=1;j<=39000;j++) 
//	str2.append("abcdefghijklmnopqrstuvwxyz");
	
//	String str=str2.toString();
	
	
	if(k>n || k>26 ||(n>1&& k==1)) System.out.println(-1);
	
	else {String result=str.substring(0,k);
	
	
	if(n<2) System.out.println("a");
	else {
	String fin=result.substring(2);
	//System.out.println("fin="+fin);
	
	int initial=n-fin.length();
	
	StringBuilder raspuns=new StringBuilder();
	
	if(initial%2==0)
	
	{for(int j=1;j<=initial/2;j++)
	raspuns.append("ab");
	raspuns.append(fin);}
	
	else {
	raspuns.append("a");
	for(int j=1;j<=(initial-1)/2;j++)
	raspuns.append("ba");
	raspuns.append(fin);
	
	
	
	
	}
		System.out.println(raspuns);
	
	}
	
}
	
	

		
		}
	
}
	

	
	
	
	
	
	