#include <iostream>
#include <math.h>
#include <algorithm>
#include <functional>
#include <vector>
#include <typeinfo>
#include <climits>
#include <ctype.h>
#include <string>
using namespace std;
#define INIT cin.tie(0); ios::sync_with_stdio(false);
#define FOR(i, a, n) for (int i = a; i < n; i++)
#define REP(i, n) for(int i = 0; i < n; i++)

int main()
{
    INIT;

    int n;
    cin >> n;
    
    int ans;
    int f = 1;
    REP(i, n){
        int x, y;
        cin >> x >> y;
        if(i == 0) ans = y - x;
        if(i != 0){
            int hoge = y - x;
            if(ans != hoge){
                f = 0;
                break;
            }
        }
    }
    if(f == 1 and ans > 0) cout << ans << endl;
    else cout << -1 << endl;

    return 0;
}