#include <iostream>
using namespace std;

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