# https://yukicoder.me/problems/no/531
import math
nm = [int(x) for x in input().split()]
n = nm[0]
m = nm[1]

if n < m:
	print(1)
elif n%2 == 1:
	print(-1)
elif math.ceil(n/2) <= m:
	print(2)
else:
	print(-1)