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

int main(){
	int n;
	cin >> n;
	vector<int> total;
	int a,b;
	for(int i = 0; i < n; i++){
		cin >> a >> b;
		int x = a + 4 * b;
		total.push_back(x);
	}
	sort(total.begin(),total.end(),greater<int>());
	int max = total[0];
	int sum = 0;
	for(int i = 1; i < n; i++){
		if((max - total[i]) % 2){
			cout << -1 << endl;
			return 0;
		}
		sum += max-total[i];
	}
	cout << sum / 2 << endl;
	return 0;
}