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: RESULTS : We evaluated 143 patients who received single - agent cisplatin ; 97 . 2 % of patients had head and neck cancer as their primary malignancy .
### Output Text:
RESULTS : We evaluated 143 patients who received single - agent <span class="chemical">cisplatin</span> ; 97 . 2 % of patients had head and neck cancer as their primary malignancy . 