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.math.*;
import java.io.*;

public class Main
	{
	public static void main(String args[]) throws IOException
		{
		InputReader in=new InputReader(System.in);
		int n=in.readInt();
		int k=in.readInt();
		if(n<k)
			{
			System.out.println("-1");
			return;
			}
		if(k==1)
			{
			if(n==1)
				System.out.println("a");
			else
				System.out.println("-1");
			return;
			}
		char A[]=new char[n];
		char c=(char)('a'+k-1);	//max character
		int index=n-1;
		for(int iter=1;iter<=k-2;iter++)
			{
			A[index]=c;
			c=(char)(c-1);
			index--;
			}
		if(index!=-1)
			{
			A[0]='a';
			for(int i=1;i<=index;i++)
				{
				if(A[i-1]=='a')
					A[i]='b';
				else
					A[i]='a';
				}
			}
		StringBuffer ans=new StringBuffer("");
		for(int i=0;i<n;i++)
			ans=ans.append(A[i]);
		PrintWriter out=new PrintWriter(System.out);
		out.println(ans.toString());
		out.close();
		}
	}

class InputReader
	{
	private InputStream stream;
	private byte[] buf=new byte[1024];
	private int curChar;
	private int numChars;
	private SpaceCharFilter filter;

	public InputReader(InputStream stream)
		{
		this.stream=stream;
		}

	public int read()
		{
		if(numChars==-1)
			throw new InputMismatchException();
		if(curChar>=numChars)
			{
			curChar=0;
			try
				{
				numChars=stream.read(buf);
				} catch (IOException e)
				{
				throw new InputMismatchException();
				}
			if(numChars<=0)
				return -1;
			}
		return buf[curChar++];
		}

	public int readInt()
		{
		int c=read();
		while (isSpaceChar(c))
			c=read();
		int sgn=1;
		if(c=='-')
			{
			sgn=-1;
			c=read();
			}
		int res=0;
		do
			{
			if(c<'0'||c>'9')
				throw new InputMismatchException();
			res*=10;
			res+=c-'0';
			c=read();
			} while (!isSpaceChar(c));
		return res*sgn;
		}

	public boolean isSpaceChar(int c)
		{
		if(filter!=null)
			return filter.isSpaceChar(c);
		return isWhitespace(c);
		}

	public static boolean isWhitespace(int c)
		{
		return c==' '||c=='\n'||c=='\r'||c=='\t'||c==-1;
		}

	public char readCharacter()
		{
		int c=read();
		while (isSpaceChar(c))
			c=read();
		return (char) c;
		}

	public interface SpaceCharFilter
		{
		public boolean isSpaceChar(int ch);
		}
	}
