# import sys; input = sys.stdin.buffer.readline
# sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	N, M = getlist()
	if N <= M:
		print(1)
	elif N <= 2 * M and N % 2 == 0:
		print(2)
	else:
		print(-1)


if __name__ == '__main__':
	main()