#include<bits/stdc++.h>
#define rep(i,a,n) for (int i = a;i < n;i++)
#define per(i,n,a) for (int i = n-1;i >= a;i--)
using namespace std;

int main() {
    int n;
    cin >> n;
    int x[n], y[n], c = -1;
    rep(i, 0, n) {
        cin >> x[i] >> y[i];
        if (c == -1) c = y[i] - x[i];
        else if (c != y[i] - x[i]) {
            cout << -1 << endl;
            return 0;
        }
        if (c <= 0) {
            cout << -1 << endl;
            return 0;
        }
    }
    cout << c << endl;
}