a, b = map(int, input().split())
bool1=False
#print(a,"to",b) a=21 b=1 i=3 j=7 out
if 1<=a and a<=20 and 1<=b and b<=20
for i in range(10):
	for j in range(10):
		#print(i)
		if i*j == a*b:
			#print("a")
			bool1=True
			#print(bool1)
			break
		else:
			#print("j={0}".format(j))
			pass
	if bool1==True:
		break
#print(bool1)
if bool1==True:
	print(a*b)
else:
	print(-1)
