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: Two patients developed device - related complications and two patients experienced abnormal weight gain .
### Output Text:
Two patients developed device - related complications and two patients experienced abnormal weight gain . 