Convert from 12 clock format to 24

using System;

namespace demoString
{
    class Program
    {
        static void Main()
        {
            var input = Console.ReadLine();

            var result = SubstringSymbols(input);
            Console.WriteLine(result);
        }

        static string SubstringSymbols(string input)
        {
            var isAm = input.Contains("AM");
            input = input.Substring(0, input.Length - 2);
            var time = string.Empty;
            for (int i = 0; i < input.Length; i++)
            {
                if (input[i] == ':') time += "";
                else time += input[i];
            }

            var firstPart = time;
            firstPart = firstPart.Substring(0, firstPart.Length - 4);
            var hour = int.Parse(firstPart);
            var secondPart = time;
            secondPart = secondPart.Substring(secondPart.Length - 4, 2);
            var minutes = int.Parse(secondPart);
            var lastPart = time;
            lastPart = lastPart.Substring(lastPart.Length - 2, 2);
            var seconds = int.Parse(lastPart);

            return PrintResult(input, isAm, hour, minutes, seconds);
        }

        static string PrintResult(string input, bool isAm, int hour, int minutes, int seconds)
        {
            if (isAm && hour == 12)
            {
                return ($"{hour - 12:d2}:{minutes:d2}:{seconds:d2}");
            }
            else if (!isAm && hour >= 1 && hour <= 11)
            {
                return ($"{hour + 12:d2}:{minutes:d2}:{seconds:d2}");
            }
            return ($"{input}");
        }
    }
}