#include <bits/stdc++.h>

using namespace std;

int main()
{
    int N; cin >> N;
    bool f = true;
    int diff;
    for (int i = 0; i < N; ++i) {
        int X, Y; scanf("%d %d", &X, &Y);

        if (f) diff = Y - X;

        if (X >= Y || diff != Y - X) {
            cout << -1 << endl;
            return 0;
        }

        f = false;
    }

    cout << diff << endl;
}