#include<iostream>
#include <math.h>
#include <string>
using namespace std;

int main(){
	int n;
	int x;
	int y;
	int a;
	int flag;
	cin >> n;
	for (int i = 0; i < n; i++){
		cin >> x;
		cin >> y;
		if (i == 0){
			a = y - x;
			if (a <= 0){
				flag = 1;
				break;
			}
		}
		else if (a != y - x){
			flag = 1;
			break;
		}
		else{
			flag = 0;
		}
	}
	if (flag == 1){
		cout << "-1" << endl;
	}
	else{
		cout << a << endl;
	}
	return 0;
}