What how do you compute the sum of 1/2 + 1/4 + 1/8 + 1/16 + 1/32 +++ 1/2N ?
it is an infinite geometric series and there is a formula for it
looking at the series sums
... 1/2, 3/4, 7/8, 15/16, 31/32...
you should see the pattern and where the sum is going