#include <iostream>
#include <vector>
using namespace std;
int main(){
	int N;
	cin >> N;
	vector<int> a(N), b(N);
	for (int i = 0; i < N; i++){
		cin >> a[i] >> b[i];
	}
	vector<int> c(N);
	for (int i = 0; i < N; i++){
		c[i] = a[i] + b[i] * 4;
	}
	int m = 0;
	for (int i = 0; i < N; i++){
		m = max(m, c[i]);
	}
	long long ans = 0;
	bool ok = true;
	for (int i = 0; i < N; i++){
		if ((m - c[i]) % 2 != 0){
			ok = false;
		} else {
			ans += (m - c[i]) / 2;
		}
	}
	if (!ok){
		cout << -1 << endl;
	} else {
		cout << ans << endl;
	}
}