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

#define N 1000
#define re(i,n) for(int i=0;i<(n);i++)
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define fe(i,n,f) for_each(i,n,f)
using namespace std;

typedef long long ll;

int main(void){
	int a,b;
	int n;
	cin>>n;
	cin>>a>>b;
	int k=b-a;
	re(i,n-1){
		cin>>a>>b;
		if(k!=b-a){
			cout<<"-1"<<endl;
			return 0;
		}
	}
	if(k<=0)cout<<"-1"<<endl;
	else cout<<k<<endl;
	return 0;
}