chopping the corners off a block

[rols]

I have a flattened cube, basically a thick rectangular block. I want to chop the top two corners off to make two right-angled 'shoulders'.

I've cut the faces so that the entire block is divided into small cubes, what I want to do is remove the top left and top right of those cubes. I've tried everything I can think of but have failed miserably.

If I take the 4 faces and 'extrude' inwards it looks like the cube has disappeared, but all that seems to have happened is the faces now overlap what's left and it's a mess. If I push the front face inwards then the rest of the side of the block tilts in, which I understand. I've tried dissolving things, collapsing things. Clearly I'm missing a piece of understanding.

If I start with the block short, divide up the top face and extrude the middle piece upwards I get the 'head and shoulders' I want, but I can't work out how to start with the full block and remove two cubes at the corners to end up with the same thing.