#define _USE_MATH_DEFINES
#include<stdio.h>
#include<string>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include<iomanip>
//#include<bits/stdc++.h>


using namespace std;



int main(){

	int n;
	cin>>n;
	int a=0;
	bool f=1;
	for(int i=0;i<n;i++){
		int x,y;
		cin>>x>>y;
		if(i==0)a=y-x;
		if(a!=y-x)f=0;
		if(a<=0)f=0;
	}
	if(!f)a=-1;
	cout<<a<<endl;
	return 0;

	}