#define _USE_MATH_DEFINES	//M_PI(π),M_SQRT2(√2)
#include <iostream>			//std::cout, std::cin
#include <string>			//std::string
#include <vector>			//std::vector
#include <valarray>			//std::valarray
#include <algorithm>		//std::sort
#include <time.h>			//localtime_s
#include <cstdlib>			//abs
#include <cmath>			//abs,std::pow,sqrt,sin,cos,round,floor,ceil
#include <fstream>			//std::ifstream
#include <iomanip>			//std::setprecision
#include <random>			//std::random(C++11)
#include <numeric>			//std::accumulate

int main(void) {

	//test用
	//std::ifstream in("test.txt");
	//std::cin.rdbuf(in.rdbuf());

	//const unsigned int MODULO = 1000000007;	//MODULO 10^9+7用

	int N, X, Y, ans;
	std::cin >> N;
	bool flg = true;

	std::cin >> X >> Y;
	ans = Y - X;
	if (ans <= 0) {
		flg = false;
	}

	if (flg) {
		int temp;
		for (int i = 0; i < N - 1; i++) {
			std::cin >> X >> Y;
			temp = Y - X;
			if (temp != ans) {
				flg = false;
				break;
			}
		}
	}

	if (flg) {
		std::cout << ans << std::endl;
	}
	else {
		std::cout << -1 << std::endl;
	}
	

}