#include<iostream>
#include<algorithm>
#include <vector>
#include<cmath>
#include<iomanip>
using namespace std;
typedef long long int lont;
int main() {
	int N;
	cin >> N;
	vector<int>veca(N);
	int x, y;
	for (int ia = 0; ia < N; ia++) {
		cin >> x >> y;
		veca.at(ia) = y - x;
	}
	int cnta = 0;
	for (int ib = 1; ib < N; ib++) {
		if (veca.at(ib - 1) != veca.at(ib)) {
			cnta++;
		}
	}
	if (!cnta && veca.at(0) > 0) {
		cout << veca.at(0) << endl;
	}
	else {
		cout << -1 << endl;
	}
}