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