#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <functional>
#include <set>
#include <sstream>
#include <map>
#include <queue>
#include <stack>

using namespace std;


int main()
{
	int n;
	cin>>n;

	long x,y;
	cin>>x>>y;

	long res=y-x;
	for(int i=0;i<n-1 && res!=-1;i++){
		cin>>x>>y;
		if(y-x!=res) res=-1;
	}

	if(res<=0) res=-1;
	cout<<res<<endl;

	return 0;
}