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

namespace Test
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            int[] len = new int[n];
            for (int i = 0; i < n; i++)
            {
                int[] x = Console.ReadLine().Split().Select(int.Parse).ToArray();
                len[i] = x[0] + x[1] * 4;
            }
            bool flag = true; long ret = 0;
            for (int i = 0; i < n; i++)
            {
                if ((len.Max() - len[i]) % 2 == 0)
                {
                    ret += (len.Max() - len[i]) / 2;
                }
                else
                {
                    flag = false;
                }
            }
            Console.WriteLine(flag ? ret : -1);
        }
    }
}