# -*- coding: utf-8 -*-
n = int(input())
x = [0] * n
y = [0] * n
for i in range(n):
    temp = list(map(int,input().split()))
    x[i] = temp[0]
    y[i] = temp[1]

ans = y[0] - x[0]

if ans > 0:
    for i in range(n):
        if y[i] - x[i] != ans:
            print("-1")
            break
    else:
        print(ans)
else:
    print("-1")