#include <iostream>
#include <cmath>
#include <string>
#include <vector>
#include <map>
#include <list>
#include <deque>
#include <algorithm>
#include <iomanip>

using namespace std;

int main() {
	int N;
	cin >> N;
	
	int ans = 0;
	for (int i = 0; i < N; i++) {
		int X = 0, Y = 0;
		cin >> X >> Y;

		if (ans != Y - X && i != 0) {
			ans = -1;
			break;
		}
		ans = Y - X;
	}

	if (ans <= 0) ans = -1;

	cout << ans << endl;
	return 0;
}