#include <iostream>
using namespace std;

int main() {
    int N;
    cin >> N;
    int x_0, y_0;
    cin >> x_0 >> y_0;
    int tmp = y_0 - x_0;
    for (int i = 0; i < N - 1; i++) {
        int x, y;
        cin >> x >> y;
        if (tmp != y - x) {
            tmp = -1;
            break;
        }
    }
    if (tmp <= 0) {
        cout << -1 << endl;
    } else {
        cout << tmp << endl;
    }
    return 0;
}