#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;
	ll a, b;
	vector<ll>v;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		cin >> a >> b;
		v.push_back(a + 4 * b);
	}
	ll MAX = v[0];
	for (int i = 1; i < n; i++)
	{
		if (v[i] % 2 != v[i - 1] % 2)
		{
			cout << -1 << endl;
			return 0;
		}
		MAX = max(MAX, v[i]);
	}
	ll sum = 0;
	for (int i = 0; i < n; i++)
	{
		sum += (MAX - v[i]) / 2;
	}
	cout << sum << endl;
	return 0;
}