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: Treatment of ovarian cancer with a combination of cis - platinum , adriamycin , cyclophosphamide and hexamethylmelamine .
### Output Text:
Treatment of ovarian cancer with a combination of <span class="chemical">cis - platinum</span> , <span class="chemical">adriamycin</span> , <span class="chemical">cyclophosphamide</span> and <span class="chemical">hexamethylmelamine</span> . 