Raster analysis following flow direction

Hello,

I am trying to obtain a raster dataset out of this analysis: for each pixel in a raster I want to substract value of the previous pixel according to the flow direction.

Lets say pixel A flows into pixel B. Pixel A has a value of 3 and pixel B has a value of 4. I'd like to get a raster where in pixel position of B I have a value of 1. For all those pixels where there is no previous pixel flowing into it, it should return a value of 0.

Any thoughts? Thx!