Inno Setup, как сделать автоопределение места установки?

[Setup]
; NOTE: The value of AppId uniquely identifies this application.
; Do not use the same AppId value in installers for other applications.
; (To generate a new GUID, click Tools | Generate GUID inside the IDE.)
AppId={{A37A6544-0ED5-4D53-B01F-88F308AC23F8}
AppName=My Program
AppVersion=1.5
;AppVerName=My Program 1.5
AppPublisher=My Company, Inc.
AppPublisherURL=http://www.example.com/
AppSupportURL=http://www.example.com/
AppUpdatesURL=http://www.example.com/
AppendDefaultDirName=no
DefaultDirName={code:MyConst}
DisableDirPage=yes
DefaultGroupName=My Program
OutputBaseFilename=setup
Compression=lzma
SolidCompression=yes
 
[Code]
const
  DefaultDrive1='D:\';
  DefaultDrive2='C:\';
  AppFolder = 'Base\';
function MyConst(Param: string): String;
 
begin
    if DirExists(DefaultDrive1 + AppFolder)then begin
    result:=DefaultDrive1 + AppFolder ;
    end;
    if DirExists(DefaultDrive2 + AppFolder)then begin
    result:=DefaultDrive2 + AppFolder;
    end;
 
end;
 
 
 
 
 
[Languages]
Name: "english"; MessagesFile: "compiler:Default.isl"
 
[Tasks]
Name: "desktopicon"; Description: "{cm:CreateDesktopIcon}"; GroupDescription: "{cm:AdditionalIcons}"; Flags: unchecked
 
[Files]
Source: "C:\Program Files\Inno Setup 5\Examples\MyProg.exe"; DestDir: "{app}"; Flags: ignoreversion
; NOTE: Don't use "Flags: ignoreversion" on any shared system files
 
[Icons]
Name: "{group}\My Program"; Filename: "{app}\MyProg.exe"
Name: "{commondesktop}\My Program"; Filename: "{app}\MyProg.exe"; Tasks: desktopicon
 
[Run]
Filename: "{app}\MyProg.exe"; Description: "{cm:LaunchProgram,My Program}"; Flags: nowait postinstall skipifsilent

[Setup]
Uninstallable=no