inputer = int(input())

for inter in range(inputer):
	x,y =map(int,input().split(" "))
	if inter>0:
		if not(tmp==(y-x)):
			result=-1
			break
	else:
		if x>=y:
			tmp = y-x
			result=tmp
		else:
			result=-1
			break

print(result)