#include <iostream>
using namespace std;

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