using System;
using System.Linq;

class Program
{
    static void Main()
    {
        int n = int.Parse(Console.ReadLine());
        long[] c = new long[n];
        for (int i = 0; i < n; i++)
        {
            string[] s = Console.ReadLine().Split();
            c[i] = long.Parse(s[0]) + long.Parse(s[1]) * 4;
        }

        for(int i=0;i< n; i++)
        {
            if ((c[i] - c[0]) % 2 != 0)
            {
                Console.WriteLine(-1);
                return;
            }
        }

        long ans = (c.Max() * n - c.Sum()) / 2;

        Console.WriteLine(ans);
    }
}