#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
using ll = long long;
using namespace std;
const int INF = 1e9;

int main() {
    int n;
    cin >> n;

    int a, b;
    cin >> a >> b;
    int ans = b - a;

    rep(i, n - 1) {
        cin >> a >> b;
        if (b - a != ans) {
            cout << -1 << endl;
            return 0;
        }
    }

    if (ans > 0)
        cout << ans << endl;
    else
        cout << -1 << endl;

    return 0;
}