#include<iostream>
#include<stdio.h>
#include<algorithm>
#include<vector>
#include<string.h>
#include<math.h>
#include<map>
#include<iomanip>
#include<queue>

const long long INF = 1e6+7;
const long long MOD = 1e9+7;
const double PI=acos(-1);

using namespace std;

int main(){
	int n;
	cin >> n;
	vector<int> x(n),y(n);
	map<int,int> mp;
	for(int i=0;i<n;i++){
		cin >> x[i] >> y[i];
		int d=y[i]-x[i];
		mp[d]=d;
	}
	vector<int> v;
	for(auto p:mp){
		v.push_back(p.second);
	}	
	if(v.size()==1 && v[0]>0){
		cout << v[0] << endl;
	}else{
		cout << -1 << endl;
	}
	return 0;
}