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

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

#処理内容
def main():
	A, B = getlist()
	GCD = gcd(A, B)
	if GCD != 1:
		print(-1)
		return

	DP = [0] * 10001
	DP[0] = 1
	for i in range(10001):
		if DP[i] == 1:
			if i + A <= 10000:
				DP[i + A] = 1
			if i + B <= 10000:
				DP[i + B] = 1

	ans = 0
	for i in range(10001):
		if DP[i] == 0:
			ans += 1

	print(ans)

if __name__ == '__main__':
	main()