The joint distribution of the amount of pollutants of a smokestack without cleaning device(y1) and with a cleaning device(y2) is given

f(y1,y2)= 1, 0</y </ 2 , 0 </ y2 </ 1, 2y2 </ y1

0 elsewhere

find the density of U= Y1-Y2.

now my problem is setting up the integral. I just can't figure out the limits of integration!