Exam 8. Bit Flipper

using System;
using System.Globalization;
using System.Threading;

public class BitFlipper
{
    public static void Main()
    {
        Thread.CurrentThread.CurrentCulture = CultureInfo.InvariantCulture;
        checked
        {
            ulong number = ulong.Parse(Console.ReadLine());

            const ulong SEQ3BITS = 7;
            for (int position = 63; position >= 2; position--)
            {
                ulong bitMask = SEQ3BITS << (position - 2);
                ulong bitValue = (number & bitMask) >> (position - 2);
                if (bitValue == 0)
                {
                    number |= bitMask;
                    position -= 2;
                }
                else if (bitValue == SEQ3BITS)
                {
                    number &= ~bitMask;
                    position -= 2;
                }
            }

            Console.WriteLine(number);
        }
    }
}