#include <map>
#include <set>
#include <list>
#include <cmath>
#include <ctime>
#include <deque>
#include <queue>
#include <stack>
#include <string>
#include <bitset>
#include <cstdio>
#include <limits>
#include <vector>
#include <climits>
#include <cstring>
#include <cstdlib>
#include <fstream>
#include <numeric>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <iomanip>
#include <cctype>
using namespace std;
typedef long long ll;

int main()
{
	int n, a, b;
	set<int>s;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		cin >> a >> b;
		if (b - a <= 0)
		{
			cout << -1 << endl;
			return 0;
		}
		if (i == 0)s.insert(b - a);
		else
		{
			if (s.find(b - a) == s.end())
			{
				cout << -1 << endl;
				return 0;
			}
		}
	}
	set<int>::iterator ite;
	ite = s.begin();
	cout << *ite << endl;
	return 0;
}