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 sys
if __name__ == '__main__':
    a = [int(x) for x in raw_input().split()]
    n = a[0]
    k = a[1]
    s = "abcdefghijklmnopqrstuvwxyz"
    if n < k or (k == 1 and n != 1):
        print -1
    else:
        for i in range(0,n):
            if n - i - 1 >= k - 2 :
                if i % 2 == 0:
                    sys.stdout.write("a")
                else:
                    sys.stdout.write("b")
            else:
                sys.stdout.write(s[k - 2 - n + i + 2])
        sys.stdout.flush()
		      		 			 	     	 	 	