289_C. Polo the Penguin and Strings_13594
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
from itertools import *


def solve(n,k):
  alpha = 'cdefghijklmnopqrstuvwxyz'
  if k>n: return -1
  if k==1: return 'a' if n==1 else -1
  if k==2: return ''.join(islice(cycle('ab'),0,n))
  if k==3: return ''.join(chain(islice(cycle('ab'),0,n-1),'c'))
  return ''.join(chain(islice(cycle('ab'),0,n-k+2),alpha[:k-2]))

n,k = map(int, raw_input().split())
print solve(n,k)