#include <bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    int x, y;
    cin >> x >> y;
    int t = y - x;
    if (t <= 0) t = -1;
    for (int i = 0; i < n - 1; i++) {
        int x2, y2;
        cin >> x2 >> y2;
        if (t + x2 != y2) t = -1;
    }
    cout << t << '\n';
    return 0;
}