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: Randomized comparison of olanzapine versus risperidone for the treatment of first - episode schizophrenia : 4 - month outcomes .
### Output Text:
Randomized comparison of <span class="chemical">olanzapine</span> versus <span class="chemical">risperidone</span> for the treatment of first - episode schizophrenia : 4 - month outcomes . 