import strutils, sequtils
proc input: auto = stdin.readLine

let N = input().parseInt
var Z: int

for i in 0 ..< N:
  var X, Y: int
  (X, Y) = input().split.map parseInt
  if Z == 0:
    Z = Y - X
  else:
    if Z != Y - X:
      Z = -1
      break
[Z, -1][int(Z <= 0)].echo