how to make item first bought, first sell and calculate cost that way

[remon]

i buy and sell 4 items no more, the problem is the items prices changes everytime i buy it.
let's say one of the items is a "sugar" ... i bought it yesterday for 1$ for 1kilo and i bought 50, then after a week 2$ for 1 kilo and i bought 50 etc...
when i sell 30, it costs me 30$ cuz it came from my 1st purchase. then i sold 60 it means 20 for 20$ from 1st purchase and 40 for 80$ from the 2nd purchase.
i need to make a sheet that calculate the cost price of every sell process based on 1st bought 1st to sell.
any ideas ???