Multiplying the two field values in crystal reports

In my crystal report I have a requirement that is,for the amount field I need to multiply two values.

ex:

quantity=10000 kg and price =90

I want to get the result as 900000!

that is in amount field i should get 900000.

I tried {com.qty}*{com.price}

it is giving me an error.

please help me out.?