#include <bits/stdc++.h>
#include "bits/stdc++.h"
#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <cmath>
#include <stack>
#include <queue>
#include <cctype>
#include <stdio.h>
#include <map>
#include <unordered_map>
#include <string.h>
#include <utility>

typedef long long ll;
const int INF = 1e8;

#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
using namespace std;

typedef pair<int, int> P;

int d[110];

int main() {

	int n = 0;
	cin >> n;

	int x, y;
	cin >> x >> y;

	int tmp = y - x;

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

	for (int i = 1; i < n; i++) {
		cin >> x >> y;

		if (y - x != tmp) {
			cout << -1 << endl;
			return 0;
		}
	}

	cout<<tmp<<endl;

	return 0;
}