Program to calculate the derivation in zoom lecture?

I have to explain an invoice in Zoom. Now I ask myself I should do the best.

I would love to "write" live. Unfortunately my laptop doesn't have a touch screen.

Is there perhaps a mobile phone app where you can type on the mobile phone and that is then displayed on the screen?

Or do you know an alternative program that is suitable for live preliminary calculations?

You can stream your screen live to your teachers or lecturers and then write down and solve the bill using Paint or Word.