cumulative tier discount

I want to create a complex offer: buy 1 product, get the second at -50%, buy 2 products = the third at -60%, and finally buy 3 products = the 4th at -70%. But these offers must be cumulative! That means the customer who buys 4 products will get the 1st without discount, the second at -50%, the third at -60%, and the fourth at -70%.