Count Substring in text

using System;

class Program
{
    public static int CountSubstring(string text, string subtr)
    {
        int count = 0;
        text = text.ToLower();
        while (text.IndexOf(subtr) != -1)
        {
            int pos = text.LastIndexOf(subtr);
            if (text.Substring(pos, subtr.Length) != "")
            {
                count++;
                text = text.Remove(pos);
            }
        }
        return count;
    }

    static void Main()
    {
        string text = "We are living in an yellow submarine. We don't have anything else. Inside the submarine is very tight. So we are drinking all the day. We will move out of it in 5 days.";
        string substring = "in";
        Console.WriteLine(CountSubstring(text, substring));
    }
}