#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cmath>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <map>
#include <functional>
#include <set>
#include <numeric>
#include <stack>
#include <utility>
//#include "util.h"

using namespace std;
typedef long long lint;
typedef pair<lint, lint> Pii;

#define PI 3.14159265358979323846
#define EPS 1e-6
#define MOD ((lint)1000000007)
#define MIN(a,b) ((a)<(b)?(a):(b))
#define MAX(a,b) ((a)>(b)?(a):(b))

int main()
{
	lint N, a, b, M = 0, eo, ans = 0;
	cin >> N;
	vector<lint> w(N);

	cin >> a >> b;
	eo = a & 1;
	w[0] = a + 4 * b;
	M = w[0];
	for (lint i = 1; i < N; i++) {
		cin >> a >> b;
		if ((a & 1) != eo) {
			cout << -1 << endl;
			return 0;
		}
		w[i] = a + 4 * b;
		M = MAX(M, w[i]);
	}
	
	for (lint i = 0; i < N; i++) {
		ans += (M - w[i]) / 2;
	}
	
	cout << ans << endl;

	return 0;
}