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.BigInteger;
import java.util.*;

public class A
{
	String line;
	StringTokenizer inputParser;
	BufferedReader is;
	FileInputStream fstream;
	DataInputStream in;
	String FInput="";
	
	void openInput(String file)
	{

		if(file==null)is = new BufferedReader(new InputStreamReader(System.in));//stdin
		else
		{
			try{
		
				
			fstream = new FileInputStream(file);
			in = new DataInputStream(fstream);
			is = new BufferedReader(new InputStreamReader(in));
			}catch(Exception e)
			{
				System.err.println(e);
			}
		}

	}
	
	void readNextLine()
	{
		try {
			line = is.readLine();
			inputParser = new StringTokenizer(line, " ");
			//System.err.println("Input: " + line);
		} catch (IOException e) {
			System.err.println("Unexpected IO ERROR: " + e);
		}	
		catch (NullPointerException e)
		{
			line=null;
			
		}
		
	}
	
	int NextInt()
	{
		String n = inputParser.nextToken();
		int val = Integer.parseInt(n);
		
		//System.out.println("I read this number: " + val);
		return val;
	}
	
	long NextLong()
	{
		String n = inputParser.nextToken();
		long val = Long.parseLong(n);
		
		//System.out.println("I read this number: " + val);
		return val;
	}
	
	double NextDouble()
	{
		String n = inputParser.nextToken();
		double val = Double.parseDouble(n);
		
		//System.out.println("I read this number: " + val);
		return val;
	}
	
	String NextString()
	{
		String n = inputParser.nextToken();
		return n;
	}
	
	void closeInput()
	{
		try {
			is.close();
		} catch (IOException e) {
			System.err.println("Unexpected IO ERROR: " + e);
		}
			
	}
	
	
	public static void main(String [] argv)
	{
		//String filePath="input.txt";
        String filePath=null;
		if(argv.length>0)filePath=argv[0];
		new A(filePath);
	}
	
	public void readFInput()
	{
		for(;;)
		{
			try
			{
				readNextLine();
				FInput+=line+" ";
			}
			catch(Exception e)
			{
				break;
			}
		}
		inputParser = new StringTokenizer(FInput, " ");
	}
	 
	public A(String inputFile)
	{
		openInput(inputFile);
		readNextLine();
    	
    	int N=NextInt();
    	int K=NextInt();
    	StringBuilder ret = new StringBuilder();
    	if(K>N||(K==1&&N>1))
    	{
    		 System.out.print("-1");
    		 return;
    	}
    	if(K==1&&N==1)
    	{
	   		 System.out.print("a");
	   		 return;
	   	}
    	for(int i=0; i<=N-K; i+=2)
    	{
    		ret.append("ab");
    	}
    	if((N-K)%2==1)ret.append("a");
    	for(int i=2; i<K; i++)
    		ret.append((char)(i+'a'));
	    	
	    System.out.print(ret);
	    	
		closeInput();			
	}
	
	

	
	
	public static void out(Object s)
	{
		try
		{
		    FileWriter fstream = new FileWriter("output.txt");
			BufferedWriter out = new BufferedWriter(fstream);
			out.write(s.toString());
			out.close();
		}catch (Exception e){
			System.err.println("Error: " + e.getMessage());
		}
	}
	

}