#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstring>
#include <cmath>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <unordered_map>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int v, w; };

ll MOD = 1000000009;
ll _MOD = 1000000009;
double EPS = 1e-10;

int main() {
	int N; cin >> N;
	vector<int> x(N);
	bool even = false, odd = false;
	int maxi = 0;
	for (int i = 0; i < N; i++) {
		int a, b; cin >> a >> b;
		x[i] = a + b * 4;
		if (x[i] % 2 == 0) even = true;
		else odd = false;
		maxi = max(maxi, x[i]);
	}
	if (even && odd) {
		cout << -1 << endl;
		return 0;
	}
	ll ans = 0;
	for (int i = 0; i < N; i++)
		ans += (maxi - x[i]) / 2;
	cout << ans << endl;
}