#include <iostream>
#include <algorithm>
#include <vector>
#include <deque>
#include <string>
#include <tuple>
#include <functional>
#include <numeric>
#include <cmath>
#include <iomanip>
#include <map>
#include <random>
//#include "toollib.h"
#define INT_MAX 2147483647
#define Loop(i, n) for(int i = 0; i < (int)n; i++)
#pragma warning (disable:4018)

using namespace std;
typedef long long int lint;

//***** Main Program *****

int main() {
	int N;
	cin >> N;
	vector<int> X(N), Y(N);
	Loop(i, N) {
		cin >> X[i] >> Y[i];
	}
	int ret = Y[0] - X[0];
	if (ret > 0) {
		Loop(i, N) {
			if (ret + X[i] != Y[i]) {
				ret = -1;
				break;
			}
		}
	}
	else {
		ret = -1;
	}
	cout << ret << endl;
	return 0;
}