#include<stdio.h>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <algorithm>
#include <sstream>
#include <iostream>
#include <string>
#include <stdio.h>

using namespace std;

int main() {

	int N, X[1005], Y[1005];
	cin >> N;
	for (int i = 0; i < N; i++) {
		cin >> X[i] >> Y[i];
	}

	int ans = 0;
	for (int i = 0; i < N; i++) {
		ans = Y[i] - X[i];
		if (i > 0) {
			if (ans != Y[i] - X[i]) {
				cout << -1 << endl;
				return 0;
			}
		}
	}

	if (ans <= 0) cout << -1 << endl;
	else cout << ans << endl;

	return 0;
}