Closing Stock = Opening + Purchases - Sales

Hi,

I want the closing stock account in assets stock to maintain the reflections from purchases and sales........just like customers reconciliation to accounts rec, i need this account to mmaintain purchases and sales value, so that i can get the accurate stock at the end of the year.... how is it possible ?? If there is no solution for the said equation, whats the alternative companies were using in ??

Thank you..

Suresh