N = int(input())
A, B = map(int, input().split())
if B - A <= 0:
	print(-1)
else:
	if N == 1:
		print(B - A)
	else:
		C = B - A
		for n in range(1, N):
			A, B = map(int, input().split())
			if B - A != C or B - A <= 0:
				C = -1
				break
		print(C)