Calculating / Populating Function - Percent Variable Showing Up NaN

I have 2 things going on here. One, the 'percent' variable is constantly showing up as NaN (not a number) no matter what numbers I give it. I suspect it is happening because the function is populating all the fields at one, which causes a 'divide by zero' scenario for the percentage field, which is last in line. Which brings me to my send thing. How to write this function correctly, as it obviously is causing problems as is, and it seems clunky. I think it should broken up into a couple different functions, but I tried that and NOTHING worked.

Here is the code, let me kow what you think:
function calculate(){
var contractsAdded = document.forms[0].contractsAdded.value;
var priceAdded = document.forms[0].priceAdded.value;
var contractsSold = document.forms[0].contractsSold.value;
var priceSold = document.forms[0].priceSold.value;
var totalAdded = (contractsAdded * priceAdded) * 100;
var totalSold = (contractsSold * priceSold) * 100;
var gainLoss = totalSold - totalAdded;
var percent = gainLoss / totalAdded;

document.forms[0].totalAddedCost.value = totalAdded;
document.forms[0].totalSoldCost.value = totalSold;
document.forms[0].gainLoss.value = gainLoss;
document.forms[0].returnPercent.value = percent;
}