#define _CRT_SECURE_NO_WARNINGS

#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <fstream>
#include <set>
#include <utility>
#include <algorithm>
#include <queue>
#include <stack>
#include <deque>
#include <numeric>
#include <sstream>
#include <list>
#include <map>
#include <stdlib.h>
#include <time.h>

#define rep(i,p,n) for(int i=(int)p;i<(int)n;i++)
#define repp(i,p,n) for(int i=(int)p;i>=(int)n;i--)
#define Sort(x) sort(x.begin(),x.end());
#define ll long long
#define INF 2000000000

using namespace std;
int main() {
	int n;
	bool st = true;
	int ans;
	cin >> n;
	rep(i, 0, n) {
		int a, b;
		cin >> a >> b;
		if (b - a <= 0) {
			cout << -1 << endl;
			return 0;
		}
		else {
			if (st) {
				ans = b - a;
				st = false;
			}
			else {
				if (ans != b - a) {
					cout << -1 << endl;
					return 0;
				}

			}
		}
	}

	cout << ans << endl;
	

	return 0;
}