#include <iostream>		//cin, cout
#include <vector>		//vector
#include <algorithm>	//sort,min,max,count
#include <string>		//string,getline, to_string
#include <cstdlib>		//abs(int)
#include <utility>		//swap, pair
#include <deque>		//deque
#include <climits>		//INT_MAX
#include <bitset>		//bitset
#include <cmath>		//sqrt, ceil. M_PI, pow, sin
#include <ios>			//fixed
#include <iomanip>		//setprecision

using namespace std;

int main() {

	int N;
	cin >> N;

	int a, b, c, d;
	if (N == 1) {
		cin >> a >> b;
		if (b <= a) {
			cout << -1 << endl;
		}
		else {
			cout << b - a << endl;
		}
	}
	else {
		cin >> a >> b;
		int ans1 = b - a;

		if (b <= a) {
			cout << -1 << endl;
			return 0;
		}

		cin >> c >> d;
		int ans2 = d - c;

		if (d <= c) {
			cout << -1 << endl;
			return 0;
		}

		if (ans1 == ans2) {
			cout << ans1 << endl;
		}
		else {
			cout << -1 << endl;
		}
	}

	return 0;

}