#include <iostream>
using namespace std;

int main(){
	int n;
	cin >> n;
	int x,y;
	cin >> x >> y;
	int check = y - x;
	for(int i = 1; i < n; i++){
		cin >> y >> x;
		if(check != y - x || check < 1){
			cout << -1 << endl;
			return 0;
		}
	}
	cout << (check < 1 ? -1 : check) << endl;
	return 0;
}