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

import sys
if n == 2:
    print(-1)
    exit()
if n == 3 and m <= 2:
    print(-1)
    exit()
if n % 2 == 1 and n > m:
    print(-1)
    exit()
if n <= m:
    print(1)
elif n <= 2 * m:
    print(2)
else:
    print(-1)