n = int(input())

x, y = [int(x) for x in input().split()]
an = y - x
if an <= 0:
  an = -1
for i in range(n - 1):
  xi, yi = [int(x) for x in input().split()]
  if an < 0:
    continue
  su = yi - xi
  if su != an:
    an = -1

print(an)