In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: OBJECTIVE : The authors compared 4 - month treatment outcomes for olanzapine versus risperidone in patients with first - episode schizophrenia spectrum disorders .
### Output Text:
OBJECTIVE : The authors compared 4 - month treatment outcomes for <span class="chemical">olanzapine</span> versus <span class="chemical">risperidone</span> in patients with first - episode schizophrenia spectrum disorders . 