using System;

​

public class Program

{

    public static void Main()

    {

        string input = "Outer[Middle[Innermost]Middle]Outer";

        string start = "[";

        string end = "]";

​

        string result = GetInnermostString(input, start, end);

        Console.WriteLine("Innermost string: " + result);

    }

​

    public static string GetInnermostString(string input, string start, string end)

    {

        int endIndex = input.LastIndexOf(end);

        if (endIndex == -1) return ""; // End marker not found

​

        int startIndex = input.LastIndexOf(start, endIndex - 1);

        if (startIndex == -1) return ""; // Start marker not found

​

        startIndex += start.Length;

​

        return input.Substring(startIndex, endIndex - startIndex);

    }

}

​