Kahneman's "add one" / "add three" exercise is his recommended way to activate System 2. If anyone has figured out how to do it using one's phone, please share

Kahneman's "add one" / "add three" exercise is his recommended way to activate System 2. If anyone has figured out how to do it using one's phone, please share. There used to be an App on Google Play store but it doesn't work on more recent versions of Android. I created a Spreadsheet based random 4-digit number prompter, which isn't bad, but I'd like better ideas if anyone has any.