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 Main{
    
    public static void main(String[]args) throws IOException{
        BufferedReader buffer = new BufferedReader(new InputStreamReader(System.in));
        String x = buffer.readLine();
        String [] y =x.split(" ");
        int n = Integer.parseInt(y[0]);
        int k= Integer.parseInt(y[1]);
 
     
      if (k==1 && n==1)
          System.out.println("a");
      else { if((k==1 && n>1) || (n<k))
             System.out.println("-1");
             else {int w =n-k+2;
                    for(int i =100; i<=w; i+=100)
                   System.out.print("abababababababababababababababababababababababababababababababababababababababababababababababababab");   

                             int r=w%100;
                             for(int i=2; i<=r; i+=2)
                              System.out.print("ab");
                             if(r%2==1)
                             System.out.print("a");
                   for(int i=2; i<k; i++)
                       {char a =(char) ('c' + i-2);
                        System.out.print(a);
                       } 
                      
                  }
           }
 
}
}