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

class Program
{
    static string InputPattern = "Input5";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("2");
            WillReturn.Add("1 1");
            WillReturn.Add("3 0");
            //1
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("4");
            WillReturn.Add("0 0");
            WillReturn.Add("0 0");
            WillReturn.Add("0 0");
            WillReturn.Add("0 0");
            //0
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("2");
            WillReturn.Add("0 0");
            WillReturn.Add("1 0");
            //-1
        }
        else if (InputPattern == "Input4") {
            WillReturn.Add("4");
            WillReturn.Add("30 0");
            WillReturn.Add("20 10");
            WillReturn.Add("10 20");
            WillReturn.Add("0 30");
            //90
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();

        var ResultSet = new List<int>();
        foreach (string EachStr in InputList.Skip(1)) {
            int[] wkArr = EachStr.Split(' ').Select(X => int.Parse(X)).ToArray();
            ResultSet.Add(wkArr[0] + 4 * wkArr[1]);
        }

        //2を法として余りが0の集合
        List<int> Mod0List = ResultSet.FindAll(X => X % 2 == 0);

        //2を法として余りが1の集合
        List<int> Mod1List = ResultSet.FindAll(X => X % 2 == 1);

        //両方とも空集合でない場合
        if (Mod0List.Count > 0 && Mod1List.Count > 0) {
            Console.WriteLine(-1);
            return;
        }

        List<int> WkList = (Mod0List.Count == 0 ? Mod1List : Mod0List);
        int wkMax = WkList.Max();
        long Answer = 0;
        WkList.ForEach(X => Answer += (wkMax - X) / 2);
        Console.WriteLine(Answer);
    }
}