#include <iostream>
#include <algorithm>
#include <vector>
#include <deque>
#include <string>
#include <tuple>
#include <functional>
#include <numeric>
#include <cmath>
#include <iomanip>
#include <map>
#include <random>
//#include "toollib.h"
#define INT_MAX 2147483647
#define Loop(i, n) for(int i = 0; i < (int)n; i++)
#pragma warning (disable:4018)

using namespace std;
typedef long long int lint;

//***** Main Program *****

int main() {
	int N;
	cin >> N;
	vector<int> a(N), b(N), sum(N);
	Loop(i, N) {
		cin >> a[i] >> b[i];
		sum[i] = a[i] + 4 * b[i];
	}
	sort(sum.begin(), sum.end(), greater<int>());
	int ret = 0;
	Loop(i, N) {
		if ((sum[0] - sum[i]) % 2 == 0) ret += (sum[0] - sum[i]) / 2;
		else {
			ret = -1;
			break;
		}
	}
	cout << ret << endl;
	return 0;
}