#include "bits/stdc++.h"

using namespace std;

const char en = '\n';
using  ll =          long long;
using ull = unsigned long long;

int main() {
	int n,x,y,result;
	cin>>n;

	cin>>x>>y;
	result=y-x;
	for(int i=1;i<n;i++){
		cin>>x>>y;
		if(y-x!=result){
			cout<<-1<<en;
			return 0;
		}
	}
	cout<<((result>0)?result:-1)<<en;

	return 0;
}