#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <iomanip>
#include <cmath>
#include <functional>
using namespace std;
#define diff(x,y) ((x > y) ? (x - y) : (y - x))
#define us(x) (x > 0) ? x : x * -1
#define lli long long int

int main() {
	int n, ans;
	cin >> n;
	vector <pair<int, int>> num(n);
	for (int i = 0; i < n; i++)
		cin >> num[i].first >> num[i].second;
	ans = num[0].second - num[0].first;
	for (int i = 1; i < n; i++) {
		if (num[i].second - num[i].first != ans) {
			cout << -1 << endl;
			return 0;
		}
	}
	if (ans < 1)
		cout << -1 << endl;
	else
		cout << ans << endl;
	return 0;
}