#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>

using namespace std;

int main() {
	int n, a, b;

	cin >> n;
	int max_w = 0;
	vector<int> w(n, 0);
	for (int i = 0; i < n; i++) {
		cin >> a >> b;
		w[i] = a + b * 4;
		max_w = max(max_w, w[i]);
	}

	long long ans = 0;
	long long diff;
	for (int i = 0; i < n; i++) {
		diff = max_w - w[i];
		if (diff % 2 == 1) {
			cout << -1 << endl;
			return 0;
		}
		ans += diff / 2;
	}

	cout << ans << endl;
	return 0;
}