Simple calculation

I want to write a calculation that will take the input from 1ST_FLOOR add 1ST_FLOOR_DEC / 10 to get a number with a decimal and subtract a value the user puts in CL_RD and place it in the box for ToBF.

not sure what I am missing

var num = this.getField ("1ST_FLOOR").value + this.getField ("1ST_FLOOR_DEC").value/10 - this.getField ("CL_RD").value;

this.getField("ToBF") = aNum