def main():
	# a,b,k = map(int,input().split())
	n,k = map(int,input().split())
	# s = int(input())
	# a = list(map(int,input().split()))
	# rsp = list(str(input()))
	# l = [list(map(int, input().split())) for _ in range(n-1)]
	# s = list(s)
 
	if n < 10 or k < 10:
		print(int(n*k))	
	else:
 		print(-1)
 
 
if __name__ == '__main__':
	main()