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.*;

public class POLOSTRING {

    /**
     * @param args
     */
    public static void main(String[] args) {
        try
        {
            BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
            //BufferedReader br=new BufferedReader(new FileReader("C:/Users/MINTOO/Desktop/Programs/Java/POLOSTRING.txt"));
            
            String s[]=br.readLine().split(" ");
            int n=Integer.parseInt(s[0]);
            int k=Integer.parseInt(s[1]);
            
            int i;
            
            if(k>n)
            {
                System.out.println("-1");
                return;
            }
            
            if(k==n)
            {
                for(i=0;i<k;++i)
                {
                    System.out.print((char)('a'+i));
                }
                System.out.println();
                return;
            }
            
            if(k==1)
            {
                System.out.println("-1");
                return;
            }
            
            if(k==2)
            {
                for(i=0;i<n/2;++i)
                {
                    System.out.print("ab");
                }
                
                if((n%2)>0)
                {
                    System.out.println("a");
                }
                else
                {
                    System.out.println();
                }
                
                return;
            }
            
            
            
            int cnab=k-2;
            
            n=n-cnab;
            
            for(i=0;i<n/2;++i)
            {
                System.out.print("ab");
            }
            
            if((n%2)>0)
            {
                System.out.print("a");
            }
            
            for(i=0;i<cnab;++i)
            {
                System.out.print((char)('a'+i+2));
            }
            
            System.out.println();
            
        }
        
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }

}
