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
n,k=map(int,input().split())
if k>n:
    print(-1)
elif n==1 and k==1:
    print("a")
elif k==1:
    print(-1)
else:
    ans=""
    k=k-2
    n=n-k
    z=99
    if n%2==0:
        ans+="ab"*(n//2)
        while(k>0):
            ans+=chr(z)
            z+=1
            k-=1
    else:
        x=99
        ans+="ab"*(n//2)
        ans+="a"
        while(k>0):
            ans += chr(x)
            x+= 1
            k-=1
    print(ans)