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.*;
public final class penguin_and_strings
{
	static Scanner sc=new Scanner(System.in);
	static PrintWriter out=new PrintWriter(System.out);
	
	static void solve(int n,int m)
	{
		char[] a=new char[26];
		int j=0;
		for(int i=97;i<=122;i++)
		{
			a[j]=(char)i;
			j++;
		}
		StringBuffer sb=new StringBuffer("");
		j=0;
		int morg=m;
		while(m>0)
		{
			sb.append(a[j]);
			j++;
			m--;
		}
		int i=1;
		boolean ans=true;
		while(sb.length()<n && ans)
		{
			if(i<sb.length()-1)
			{
				inner:
				for(j=0;j<26;j++)
				{
					if((a[j]<sb.charAt(i) || a[j]<sb.charAt(i+1)) && (a[j]!=sb.charAt(i) && a[j]!=sb.charAt(i+1))) 
					{
						sb.insert(i+1,a[j]);
						i++;
						break inner;
					}
					else if(a[j]>sb.charAt(i) && a[j]>sb.charAt(i+1))
					{
						break inner;
					}
				}
			}
			else
			{
				inner:
				for(j=0;j<26;j++)
				{
					if(j<morg && a[j]!=sb.charAt(i))
					{
						sb.insert(i+1,a[j]);
						i++;
						break inner;
					}
					else 
					{
						if(j>=morg)
						{
							ans=false;
							break inner;
						}
					}
				}
			}
		}
		if(sb.length()!=n)
		{
			out.println("-1");
		}
		else
		{
			out.println(sb.toString());
		}
	}
	
	public static void main(String args[]) throws Exception
	{
		int n=sc.nextInt();
		int m=sc.nextInt();
		if(m>n)
		{
			out.println("-1");
		}
		else
		{
			if(n<=1)
			{
				out.println("a");
			}
			else
			{
				if(n==m)
				{
					int i=97;
					while(m>0)
					{
						out.print((char)i);
						i++;
						m--;
					}
					out.println("");
				}
				else
				{
					if(m==1)
					{
						out.println("-1");
					}
					else
					{
						solve(n,m);
					}
				}
			}
		}
		out.close();
	}
}