using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        //Console.WriteLine(CalculateTermIndex("p:0"));

​

    }

​

    public const string RefPrefix = "p:";

    public static readonly IReadOnlyDictionary<char, int> Symbols = "0123456789abcdefghijklmnopqrstuvwxyz".Select((x, i) => (x, i)).ToDictionary(x => x.x, x => x.i);

​

    public static int? CalculateTermIndex(string base36reference)

    {

        if(base36reference.StartsWith(RefPrefix) && base36reference.Length > RefPrefix.Length)

        {

            int result = 0;

​

            int i = RefPrefix.Length;

            while (i < base36reference.Length)

            {

                if(!Symbols.TryGetValue(base36reference[i], out int index))

                {

                    return null;

                }

​

                result += (int)Math.Pow(Symbols.Count, base36reference.Length - RefPrefix.Length - 1) * index;

                i++;

            }

​

            return result;

        }

​

        return null;

    }

​

}

​