public class ReadingBooks
{
    public int countBooks(string[] parts)
    {
        int res = 0;
        int idx = 0;
        while (idx + 2 < parts.Length)
        {
            int delta = 1;
            if (parts[idx] != parts[idx + 1] && parts[idx + 1] != parts[idx + 2] && parts[idx] != parts[idx + 2])
            {
                res++;
                delta = 3;
            }
            idx += delta;
        }
        return res;
    }
}