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.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import javafx.util.Pair;

public  class CodeForce {
static	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
static StringBuilder sb = new StringBuilder();

	static boolean flag = false;
	 
	static int next(String [] sr,int ind)
	{
		return Integer.parseInt(sr[ind]);
	}

	public static void main(String[] args) throws IOException {
		
String[] sr=br.readLine().split(" ");
int n=next(sr,0);
int k=next(sr,1);
if(n==k&&k==1)
{System.out.println('a');
return;}
if(n<k)
{
	System.out.println("-1");
	return;}
if(k==1&&n>=2)
	{System.out.println("-1");
return;}
String ss="cdefghijklmnopqrstuvwxyz";
if((n-k+2)%2!=0)
{int p=(n-k+2)/2;
for(int i=0;i<p;i++)
	sb.append("ab");
sb.append("a");

sb.append(ss.substring(0,k-2));
System.out.println(sb.toString());
	
	
	
	}
else
{
	int p=(n-k+2)/2;
	for(int i=0;i<p;i++)
		sb.append("ab");
	

	sb.append(ss.substring(0,k-2));
	System.out.println(sb.toString());
		
		
		
	}











	
}


}

		
		
		
		
		
		
		
		
		

		
		
