# coding=utf-8:

n = int(input())
numList = list()

for i in range(n):
    numList.append(list(map(int, input().split())))

sabunA = 0
sabunB = 0
count = 0
j, k = 0, 1

if n == 1:
    sabunA = numList[j][1] - numList[j][0]
    if sabunA < 0:
        print(-1)

elif n >= 2:
    while j == k - 1 and k != len(numList):
        sabunA = numList[j][1] - numList[j][0]
        sabunB = numList[k][1] - numList[k][0]
        if sabunA == sabunB:
            count += 1
            j += 1
            k += 1
        else:
            j += 1
            k += 1
    if count == len(numList) - 1 and sabunA > 0:
        print(sabunA)
    else:
        print(-1)