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
#!/usr/bin/python

from string import ascii_lowercase as al
from collections import deque

def ir():
    return int(raw_input())

def ia():
    line = raw_input()
    line = line.split()
    return map(int, line)

def solve():
    d = dict(enumerate(al))
    ans = []
    c, p = 'a', 'b'
    for i in range(n):
        ans.append(c)
        c, p = p, c
    for i in range(k-2):
        if n - 1 - i < 0: print -1; return
        if k - i - 1 < 0 or k - i - 1 >= len(d): print -1; return
        ans[n-1-i] = d[k - i - 1]

    if len(set(ans))!=k: print -1; return
        
    print ''.join(ans)

n, k = ia()
solve()
