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: Thirty - five lamivudine - na ve HBV infected patients with or without HIV co - infection were studied : 15 chronic HBV mono - infected patients and 20 HBV - HIV co - infected patients .
### Output Text:
Thirty - five <span class="chemical">lamivudine</span> - <span class="chemical">na</span> ve HBV infected patients with or without HIV co - infection were studied : 15 chronic HBV mono - infected patients and 20 HBV - HIV co - infected patients . 