need help calcutating sum based on two criteria

i am doing a forgien exchange investment worksheet and i want a statment to caulate the sum base on two criteia.

e.g find the sum base on "USD" and "Buy" and the answer should be 4000 (3000+1000) see below.

the cells look like this

column A = currency
column B = transaction record (buy/sell)
column C= amount involved

A B C
Currency transaction amount
1 Yen BUY 100000
2 GBP Sell 2000
3 USD Buy 1000
4 USD Buy 3000
5 HKD Sell 5000
6 USD Sell 600

Thx in advance