using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    public static void Main()
    {
        var count = int.Parse(Console.ReadLine());

        var inputs = new List<int>();
        for (var i = 0; i < count; i++){
            var l = Console.ReadLine().Split(' ').Select(s => int.Parse(s)).ToList();
            inputs.Add(l[0] + l[1] * 4);
        }

        if (inputs.All(x => x % 2 == inputs.Max() % 2))
            Console.Write("{0}", inputs.Sum(x => inputs.Max() - x) / 2);
        else
            Console.Write("{0}", -1);

        return;
    }
}