Tricky Integration Problem

At my job we deal with "steel coils".  These are huge roles of sheet metal that can weigh up to 30+ tons.  Think giant paper towel roll made of steel.  An interesting question I had, but cannot seem to mathematically figure out on my own is the following:  Given a coil with exterior diameter of 1000mm, interior diameter of 250mm, and a metal thickness of 2.5mm, how long will the sheet of metal be if you unroll it completely?  The answer, according to an online calculator I found it 294.52 meters.  I know that to figure this out, I'll need to integrate the various layers of the coil, but I can't seem to figure out the right set-up.  Can anyone help?