Five div boxes, two on a row and the fifth one on top of the four - how do I do this in css?

This is what I have to do and I have no idea how to do it, i can create the four divs, but the fifth one is giving me trouble.

Here is what I need to do: create five divs, four of them need to be grouped in two, and the fifth one needs to be placed on top of the four divs. I have attached an image so you can better understand.

click here to see image