#include <iostream>	//cin, cout
#include <vector>	//vector
#include <algorithm> //sort,min,max
#include <string>	//string
#include <ios>		//fixed
#include <iomanip>	//setprecision
#include <utility> //swap
#include <cstdlib>	//abs(int)
#include <cmath>	//sqrt
#include <sstream>	//stringstream,getline

using namespace std;

int main() {

	int N, X, Y;
	cin >> N >> X >> Y;

	int ans = -1;
	if (Y > X) {
		ans = Y - X;
	}

	for (int i = 1; i < N; i++) {
		cin >> X >> Y;
		if (Y - X != ans) {
			ans = -1;
		}
	}

	cout << ans << endl;

	return 0;

}