I dnon't know if my maths are right but this logic seems to follow: one square = 1 grain, 2 = 3 grains, 3=7 grains in totlal which in each case works out as (2^Sqares)-1. By that reckoning we are looking at 2 to the power 64 - 1 which I can't calculate exactly using Excel and my C compiler is being stubborn.

We had the same number of digits though. by my reckoning which may well be wrong, we are looking around 18,446,744,073,709,600,000

Jon