Custom form in Smartformer for Joomla

Hi, I need a custom form built for Smartformer in Joomla. The form will need to calculate input from user. 5 Input Fields (A) A (x) B = C A (x) B = D A (x) B = E A (x) B = F A (x) B = G Variable (B) must be changeable and built into form so when it loads Variable (B) is already there. Then Total Field of : C+D+E+F+G Fairly Simple form but I need it to be built using Smartformer so I can then load and customize the rest myself. I'm just having trouble using multiplication in javascript.