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=list(map(int,input().split()))
if n<m or (m==1 and n!=1):
    print(-1)
elif n==1 and m==1:
    print('a')    
else:    
    d = {'a':1,'b':2,'c':3,'d':4,'e':5,'f':6,'g':7,'h':8,
        'i':9,'j':10,'k':11,'l':12,'m':13,'n':14,'o':15,'p':16,'q':17,
        'r':18,'s':19,'t':20,'u':21,'v':22,'w':23,'x':24,'y':25,'z':26
        }
    dd=dict((v,k) for k,v in d.items())
    a=m-2
    s='ab'*((n-a)//2)
    b=[s]
    if n-len(s)>a:
        b.append('a')
        
    for i in range(3,3+a):
        b.append(dd[i])
    
    print("".join(b))      

