Excel IF "TEXT" THEN OPERATION

I need an IF function with the condition that if the value of the cell A1 is the letter "G", then the value of the cell B1 (which is in the accounting number format $) will become negative (multiply by -1) but then if the cell A2 (and so on) contains the letter "I", the number will remain positive.

I have the following but it's not working

IF(A1="G",B1*-1,B1)