#include <iostream>
using namespace std;
int main() {
    int n, i, a, b, t;
    cin >> n;
    for (i = 0; i < n; i++) {
        cin >> a >> b;
        if (i == 0) {t = b - a;}
        else if (b - a != t) {t = -1; break;}
    }
    if (t < 1) {t = -1;}
    cout << t << endl;
    return 0;
}