Computing Average FIGURED OUT

[twisted]

I have two text boxes that are blank until I click a btnCalculate.
I need to get an average of those two text boxes. The text boxes are named SP = txtSalepersons.Text and TS = txtTotalSales.Text
I need to get the TS / SP. But when I write:

txtAverageSales.Text = TS / SP

it doesn't work because the first time there are no values to average. After the first time it works like a charm b/c values are in the box.

So basically the first time I click it I just want it to display TS and then after the first time display TS / SP. Please help, I think I might need an if then statement.