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

namespace Programming
{
    class Program
    {
        static void Main(string[] args)
        {
            //入力
            int N = int.Parse(Console.ReadLine());

            Int64[] a = new Int64[N];
            string[] str = new string[2];

            for (int i = 0; i < N; i++)
            {
                str = Console.ReadLine().Split(' ');
                a[i] = int.Parse(str[0]) + int.Parse(str[1]) * 4;
            }

            Int64 max = a.Max();     //最長文字数
            Int64 cnt = 0;

            foreach (var item in a)
            {
                if ((max - item)%2 == 1)
                {
                    Console.WriteLine(-1);
                    return;
                }
                else
                {
                    cnt += (max - item) / 2;
                }
            }

            Console.WriteLine(cnt);
        }
    }
}