#include <iostream>
using namespace std;

int main(void)
{
	int N, X, Y, ans;
	bool error = false;
	cin >> N;
	for(int i = 0; i < N; i++) {
		cin >> X >> Y;
		if(i == 0) ans = Y - X;
		if(Y - X != ans) {
			error = true;
		}
		else if(ans < 1) {
			error = true;
		}
	}
	if(error) {
		cout << "-1" << endl;
	}
	else {
		cout << ans << endl;
	}
}