#include <iostream>
#include <vector>
#include <string>
#include <array>
#include <algorithm>
#include <set>
using namespace std;

int main(){
	int n;
	cin >> n;
	long long int m_size = 0;
	long long int a,b;
	long long int sum = 0;
	for(int i = 0; i < n; i++){
		cin >> a >> b;
		long long int x = a + 4 * b;
		if(x%2 != m_size%2 and i != 0){
			cout << -1 << endl;
			return 0;
		}
		sum += x;
		m_size = max(m_size,x);
	}
	cout << (m_size * n - sum) / 2 << endl;
	return 0;
}