#include<stdio.h>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <algorithm>
#include <sstream>
#include <iostream>
#include <string>
#include <stdio.h>

using namespace std;

int main() {

	int N, a, b;
	long long sum[1005] = {};

	cin >> N;
	for(int i = 0; i < N; i++) {
		cin >> a >> b;
		sum[i] = a + 4 *b;
	}

	sort(sum, sum+N);

	long long cnt = 0;
	for (int i = 0; i < N-1; i++) {
		if ( (sum[N - 1] - sum[i]) % 2 == 0) {
			cnt += (sum[N - 1] - sum[i])/2;
		}
		else {
			cout <<-1<< endl;
			return 0;
		}
	}

	cout << cnt << endl;

	return 0;
}