#coding: UTF-8

import sys
import re
import itertools
from math import log
from collections import deque

### defs ###


### main ###
N = int(sys.stdin.readline())
XY = []
for _ in range(N):
	x,y = map(int,sys.stdin.readline().split())
	XY.append((x,y))

m = XY[0][1]-XY[0][0]
if (len(list(filter(lambda x:x[1]-x[0]==m, XY)))==N and m > 0):
	print(m)
else:
	print(-1)