# -*- coding:utf-8 -*-
import functools

if __name__ == '__main__':
    n = int(input())
    l = []
    for _ in range(n):
        l.append(tuple(map(int, input().split())))
    l = tuple(map(lambda x: x[1] - x[0], l))
    print(l[0] if functools.reduce(lambda x, y: x == y, l) >= 0 else -1)