#include "bits/stdc++.h"
#include <unordered_set>

#define _CRT_SECURE_NO_WARNINGS

#define REP(i, n) for(decltype(n) i = 0; i < (n); i++)
#define REP2(i, x, n) for(decltype(x) i = (x); i < (n); i++)
#define REP3(i, x, n) for(decltype(x) i = (x); i <= (n); i++)
#define RREP(i, n) for (decltype(n) i = (n) - 1; i >= 0; i--)

#define ALL(a) (a).begin(),(a).end()
#define SORT(c) sort((c).begin(),(c).end())
#define DESCSORT(c) sort(c.begin(), c.end(), greater<int>())

using namespace std;

const int INF = 1000000000;
const double PI = 3.1415926535897932384626433832795;

// N, E, S, W
const int dx[4] = { 1, 0, -1, 0};
const int dy[4] = { 0, 1, 0, -1};

using LL = long long int;
using LD = long double;
using pii = pair<int, int>;
using pll = pair<LL, LL>;
using pdd = pair<double, double>;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vl = vector<LL>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vd = vector<double>;
using vs = vector<string>;
using vb = vector<bool>;

//==============================================

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	int n;
	cin >> n;

	vl A(n);

	LL a, b;
	REP(i, n) {
		cin >> a >> b;
		A[i] = a + b * 4;
	}

	LL lMax = -1;
	REP(i, n - 1) {
		if (A[i] % 2 != A[i + 1] % 2) {
			cout << "-1" << "\n";
			return 0;
		}

		lMax = max(lMax, A[i]);
	}

	lMax = max(lMax, A[n - 1]);

	LL sum = 0;
	REP(i, n) sum += (lMax - A[i]) / 2;

	cout << sum << "\n";
	return 0;
}