sequences and series - Easy summation question: $S= 1-frac{1}{2}+frac{1}{4}-frac{1}{8}+frac{1}{16}cdots$

While during physics I encountered a sum I couldn't evaluate:

$$S= 1-\frac{1}{2}+\frac{1}{4}-\frac{1}{8}+\frac{1}{16}\cdots$$
Is there a particular formula for this sum and does it converges?