// コピーして使う！
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vll = vector<long long>;

int main() {
    ll n;
    cin >> n;
    vll x(n), y(n);
    for (ll i = 0; i != n; i++) {
        cin >> x.at(i) >> y.at(i);
    }
    ll a;
    a = y[0] - x[0];
    for (ll i = 1; i != n; i++) {
        if (a != y[i] - x[i]) {
            a = -1;
            break;
        }
    }
    cout << a << endl;

    system("pause");
}