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

def f(n, m):
  if m >= n:
    return 1
  elif n % 2 == 0:
    if m >= n // 2:
      return 2
    else:
      return -1
  # elif not n % 2 == 0:
  #   if m >= n // 2 + 1:
  #     return 2
  #   else:
  #     return -1
  else:
    return -1
print(f(n, m))