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

int main() {
    int n; cin >> n;
    int prev;
    for (int i = 0; i < n; i++) {
        int x, y; cin >> x >> y;
        int a = y-x;
        if (i != 0 && a != prev || a < 1) {
            puts("-1");
            return 0;
        }
        prev = a;
    }
    cout << prev << endl;
}