#include "bits/stdc++.h"
using namespace std;
using Graph = vector<vector<int>>;
typedef long long ll;
typedef pair<ll, ll> P;
#define rep(i,n) for (ll i = 0; i < (n); ++i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
const long long mod = 1e9 + 7;

ll gcd(ll a, ll b) {
	return b ? gcd(b, a % b) : a;
}

ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}

int main() {
	ll n;
	cin >> n;

	map<ll, ll> m;
	ll ans=0;

	rep(i, n) {
		ll x, y;
		cin >> x >> y;
		ll num = y-x;
		m[num]++;
	}

	if (m.size() != 1) {
		cout << -1 << endl;
		return 0;
	}

	for (auto p : m) {
		if (p.first <= 0) {
			cout << -1 << endl;
			return 0;
		}
		cout << p.first << endl;
		return 0;
	}
	
}