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.ArrayList;
import java.util.Scanner;


/**
 *
 * @author DHA
 */

public class JavaApplication4 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) throws IOException {
        // TODO code application logic here
       Scanner scan =new Scanner(System.in);
       int n=scan.nextInt();
       int k=scan.nextInt();
       char a=97;
       
       if(n>k)
       {
        if(k==1)
               System.out.println(-1);
       else{  
           for (int j = 0; j < n-(k-2); ) {
                System.out.print(a);
                j=j+1;
                if(j==n-(k-2))
                    break;
                System.out.print((char)(a+1));
                j=j+1;
           }
            for(int j=0;j<(k-2);j++)
                System.out.print((char)(a+2+j));}
       }
       else if(n==k)
       {
           for(int j=0;j<k;j++)
                System.out.print((char)(a+j));
       }
       else
            System.out.println("-1");
        
//       int r,l,sum=0;
//        for (int i = 0; i < n; i++) {
//            r=scan.nextInt();
//            l=scan.nextInt();
//          sum+= ((l-r)+1);
//        }
//        
//        int m=0;
//        while((sum%k)!=0)
//        { m++;
//        sum++;
//        }
//        System.out.println(m);
//        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
//      
         
}}
