#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <cmath>
#include <climits>
#include <iomanip>
#include <set>
#include <map>
using namespace std;
typedef long long ll;
int main(){
    ll n;
    cin >> n;
    ll res = -1;
    for(int i = 0;i < n;i++){
        ll a,b;
        cin >> a >> b;
        if(i == 0)res = b - a;
        else if(res != b - a || b - a <= 0){
            cout << -1 << endl;
            return 0;
        }
    }
    cout << res << endl;
}