#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <stdio.h>
#include <cstring>
#include <queue>
#include <stack>
#include <functional>
#include <math.h>
#include <iomanip>
using namespace std;
#define Would
#define you
const int INF = 999999999;
const int MOD = 1e9 + 7;

int main() {
	int n, a, b, mon;
	cin >> n;
	for (int i = 0; i < n; ++i) {
		cin >> a >> b;
		if (a - b < 0) {
			cout << -1 << endl;
			return 0;
		}
		if (i == 0) { mon = a - b; }
		else {
			if (a - b != mon) {
				cout << -1 << endl;
				return 0;
			}
		}
	}
	cout << mon << endl;
}