import math

a, b = [ int(v) for v in input().split() ]

x = a + b

xsqrt = math.ceil((x*1.0)**0.5)

ans = -1
for i in range(1,xsqrt+1):
	if x % i == 0:
		if a != i and b != i and (a + i) % b == 0 and (b + i) % a == 0:
			ans = i
			break

print(ans)