#!/usr/bin/env python
#coding:utf8

def read():
    xyList = []
    for i in range(input()):
        xyList.append(map(int, raw_input().split()))
    return xyList


def work(xyList):
    ans = -1
    for (x, y) in xyList:
        if x >= y:
            print -1
            return
        elif ans != -1 and ans != y - x:
            print -1
            return
        else:
            ans = y - x
    print ans


if __name__ == "__main__":
    work(read())