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,m=input().split(" ")
n=int(n)
m=int(m)


str=""
list=[]
for i in range(0,n):
    list.append("")
list[0]="a"
if(m==n ):
    j=0
    d=ord("a")
    while(j<n):
        list[j]=(chr(d+j))
        j=j+1

    c = str.join(list)
    print(c)
elif(m>n or  m==1):
    print("-1")

else:
    d=ord("a")

    i=1
    while(i<n-m+2):

        list[i]="b"

        i=i+2


    j=2
    while(j<n-m+2):

        list[j]="a"
        j=j+2




    j=2
    i=n-m+2

    while(i<n):

        list[i]=(chr(d+j))
        j=j+1
        i=i+1

    c=str.join(list)
    print(c)