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

int main() {
    int q, x, y;
    cin >> q >> x >> y;
    const int ans = y - x;
    bool ok = (ans > 0);
    while (--q) {
        cin >> x >> y;
        ok &= (ans + x == y);
    }
    cout << (ok ? ans : -1) << endl;
    return 0;
}