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: Interaction between warfarin and levofloxacin : case series .
### Output Text:
Interaction between <span class="chemical">warfarin</span> and <span class="chemical">levofloxacin</span> : case series . 