#include <bits/stdc++.h>
using namespace std;

int main() {
	long long a;
	cin >> a;
	long long n, m, cc;
	vector<long long> v(a);
	for (int i = 0; i < a; ++i) {
		cin >> n >> m;
		v[i]=n + 4 * m;
	}
	sort(v.begin(), v.end());

	cc = v[a - 1];
	//cout<<cc;
	long long ans = 0;
	for (int i = 0; i < a; ++i) {
		if (cc == v[i]) {
			continue;
		} else if ((cc % 2) == (v[i] % 2)) {
			ans = ans + cc - v[i];
			continue;
		}
		cout << -1 << endl;
		return 0;
	}
	cout << ans/2 << endl;
}