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.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class PoloC {

    /**
     * @param args
     */
    public static void main (String[] args) {
       
            new PoloC().solve();
    }
    
    public void solve()
    {
      StringBuilder string = new StringBuilder();
      Scanner s = new Scanner(System.in);
      int n = s.nextInt();
      int k = s.nextInt();
      if(n<k)
      {
          System.out.println("-1");
          return;
      }
      if(n==1&&k==1)
      {
          System.out.println("a");
          return;
      }
      if(n>1&&k==1)
      {
          System.out.println("-1");
          return;
      }
      int ending = k -2;
      int cycle = n-ending;
      int repeat = cycle/2;
      for(int i =0; i<repeat; i++){
          string.append("ab");
      }
      if(cycle%2!=0)
      {
          string.append("a");
      }
    
      for(int i = 1; i<=ending; i++)
      {
          char letter = (char)((int)'b'+i);
          string.append(letter);
      }
      System.out.println(string);
    
       
    }
    
   

}