Excel/Google Sheets: How to have a conditional multiplier

I have a multisheet that I need to do the following:

If 'UserInput'!A9=1, multiply the following formula's output by 1.5: 'Data'!A1*(('Data'!A2*'Data'!A3)+'Data'!A4)

[The sheet and cell names aren't particularly relevant.]

In other words, how could I add a conditional multiplier to an already-existing formula, without complicating user input?