#include <iostream>
using namespace std;

int main(){
	long long ans = 0,n,count = 0;
	cin >> n;
	long long a[n],b[n];
	for(int i = 0;i < n;i++){
		cin >> a[i] >> b[i];
		count = max(count,a[i] + 4 * b[i]);
	}
	for(int i = 0;i < n;i++){
		if(count - (a[i] + 4 * b[i]) & 1){
			cout << -1 << endl;
			return 0;
		}
		ans += (count - (a[i] + 4 * b[i])) / 2;
	}
	cout << ans << endl;
}