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 sys;

def main():

	nk=map(int,raw_input().strip().split(" ",2));
	n=nk[0];
	k=nk[1];
	sal="cdefghijklmnopqrstuvwxyz";
	sr="ab";
	if n<k:
		print "-1";
	elif k==1 and n!=1:
		print "-1";
	elif k==1 and n==1:
		print "a";
	elif n==k:
		print sr+sal[0:k-2];
	else:
		noudl=n-k;
		ans=sr;
		i=0;
		while(noudl>0):
			if i%2==0:
				ans=ans+sr[0];
			else:
				ans=ans+sr[1];
			i+=1;
			noudl-=1;
		ltf=n-len(ans);
		ans=ans+sal[0:ltf];
		print ans;
			
		
if __name__=="__main__":main()
