#include <iostream>
using namespace std;
int main(void) {
	long long  
		n, a, b, len, prev_a, 
		max_len = 0, sum = 0;
	cin >> n;
	bool ok = true;
	for (int i = 0; i < n; ++i) {
		cin >> a >> b;
		if (i > 0) if (a % 2 != prev_a % 2) 
			ok = false;
		len = a + 4 * b;
		sum += len;
		max_len = max(max_len, len);
		prev_a = a;
	}
	if (ok == 0) cout << -1 << endl;
	else {
		cout << (max_len * n - sum) / 2 << endl;
	}
	return 0;
}