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

namespace YukiCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            var na = new int[N];
            for (int i = 0; i < N; i++)
            {
                var va = Console.ReadLine().Split().Select(int.Parse).ToArray();
                na[i] = va[0] + va[1] * 4;
            }
            int nMax = na.Max();
            int nn = (nMax * N - na.Sum()) / 2;
            if(na.Any(c=> (nMax-c)%2==1))       Console.WriteLine(-1);
            else                        Console.WriteLine(nn);
            Console.ReadLine();
        }
    }
}