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 string
basestr = string.lowercase

def generate_abs(n):
    if n == 1:
        return 'a'
    x, y = divmod(n, 2)
    return 'ab' * x + (y and 'a' or '')


def solve(n, k):
    if n < k:
        return -1
    if k == 1 and n != 1:
        return -1
    if k <= 2:
        return generate_abs(n)

    m = n - k + 2
    abs = generate_abs(m)
    suffix = basestr[2:k]
    return abs + suffix


def test():
    print generate_abs(2)
    print generate_abs(1)
    print generate_abs(10)
    print generate_abs(7)
    print solve(7,4)
    print solve(4,7)
    print solve(8,2)

def go():
    n, k = [int(x) for x in raw_input().split()]
    print solve(n, k)


if __name__ == '__main__':
    go()

