Could not deduce (Num int) from the context ()

[edwin ancaer]

Hello, 

I'vo got the following code in leksah: 

----------------------------------------------------------------------------
--
-- Module      :  Main
-- Copyright   :
-- License     :  AllRightsReserved
--
-- Maintainer  :
-- Stability   :
-- Portability :
--
-- |
--
-----------------------------------------------------------------------------

module Main (


) where
firstfunction :: int -> int 
firstfunction  a =  a * a

main = putStrLn (show ( firstfunction 5 ))


Could you please explain why I get the error below, and what does it
mean? I copied the function definition from Yet another Haskell
tutorial, page 45 (it's called square there).
 

src/Main.hs:20:20:
    Could not deduce (Num int) from the context ()
      arising from a use of `*' at src/Main.hs:20:20-24
    Possible fix:
      add (Num int) to the context of
        the type signature for `firstfunction'
    In the expression: a * a
    In the definition of `firstfunction': firstfunction a = a * a