using System;

using System.Globalization;

public class Program

{

    public static void Main()

    {

        var dateTime = DateTime.ParseExact("26.06.2021 10:23:09", "dd.MM.yyyy HH:mm:ss", CultureInfo.InvariantCulture).ToString("yyyyMMddHHmmss");

        Console.WriteLine(dateTime);

    }

}