#include <bits/stdc++.h>
using namespace std;

int main()
{
int i;
int N;
int X;
int Y;
int iChk;

	cin >> N;
	cin >> X;
	cin >> Y;

	iChk = Y - X;
	if( iChk <= 0 )
	{
		cout << - 1 << endl;

		return 0;
	}

	for( i = 1; i < N; i++ )
	{
	cin >> X;
	cin >> Y;
		if( iChk != Y - X )
		{
			cout << - 1 << endl;

			return 0;
		}
	}

	cout << iChk << endl;

	return 0;
}