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

int main() {
	int N;
	cin >> N;
	int lmax = 0;
	int lmin = 99999999;
	for (int i = 0; i < N; i++)
	{
		int a, b;
		cin >> a >> b;
		int len = a + b * 4;
		if (i != 0){
			if (lmax % 2 != len % 2){
				cout << -1 << endl;
				return 0;
			}
		}
		lmax = max(len, lmax);
		lmin = min(len, lmin);
	}
	cout << (lmax - lmin) / 2 << endl;
}