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

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

        string[] ab;
        int[] abCntList = new int[N];
        bool isKisuu = true;

        ab = Console.ReadLine().Split(' ');
        abCntList[0] = int.Parse(ab[0]) + 4 * int.Parse(ab[1]);
        isKisuu = (abCntList[0] % 2 != 0 ? true : false);

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

            if (isKisuu ? abCntList[i] % 2 == 0 : abCntList[i] % 2 != 0)
            {
                Console.WriteLine(-1);
                return;
            }
        }

        int maxCnt = abCntList.Max();
        long ans = 0;

        for (int i = 0; i < N; i++)
        {
            ans += (maxCnt - abCntList[i]) / 2;
        }

        Console.WriteLine(ans);
    }
}