#include <bits/stdc++.h>
#define PI 3.14159265359
#define NIL -1
using namespace std;
const int64_t MOD = 1e9 + 7;

int main() {
	int N;
	cin >> N;

	int max = 0;
	vector<int> v(N);
	for (int i = 0; i < N; i++) {
		int a, b;
		cin >> a >> b;

		int n = a + 4 * b;
		if (n > max) max = n;
		v.at(i) = n;
	}

	bool bl = true;
	int ans = 0;
	for (int i = 0; i < N; i++) {
		int n = max - v.at(i);

		if (n % 2) {
			bl = false;
			break;
		}
		else {
			ans += n / 2;
		}
	}

	cout << (bl ? ans : -1) << endl;
}