# -*- coding: utf-8 -*-
N = input()
B = 0
for i in range(0, N):
    X, Y = map(int, raw_input().split())
    A  = Y - X
    if A != B and 0 < i:
        print -1
        exit()
    if A <= 0:
        print -1
        exit()
    B = A
print A