<s>[INST] # Define the sequence $a_n$ such that $a_n = \frac{1}{n}$ if $n$ is odd and $a_n = -\frac{1}{n}$ if $n$ is even. Find the sum of the elements of the set $S = \{a_1, a_3, a_5, ..., a_{99}\}$.[/INST]