import sys
n = int(input())
a,b = map(int,input().split())
s = b-a
if n == 1 or s < 0:
	print(max(-1, b-a))
	sys.exit()
for i in range(n-1):
	a,b = map(int,input().split())
	if s != b-a:
		print(-1)
		sys.exit()
print(s)