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

int main() {
	int N;
	cin >> N;
	long long a[N], b[N], c[N];
	for (int i = 0; i < N; ++i) {
		cin >> a[i] >> b[i];
		c[0] = b[0] - a[0];
		if (c[0] <= 0) {
			cout << -1 << endl;
			return 0;
		}
		c[i] = b[i] - a[i];
		if (c[0] != c[i]) {
			cout << -1 << endl;
			return 0;
		}
	}
	cout<<c[0]<<endl;
}