The given expression is a geometric series with the first term as 1/2 and the common ratio as -1/2.
Using the formula for the sum of an infinite geometric series with |r| < 1, we can find that the sum of the series is:
S = a/(1-r) = (1/2)/(1-(-1/2)) = (1/2)/(3/2) = 1/3
Therefore, the expression (1/2 - 1/4 - 1/8 - 1/16 + ...) - 1 simplifies to:
1/3 - 1 = -2/3
So, the answer is -2/3.