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: The average age of patients was 35 . 6 + / - 28 . 3 years , and 62 % were males .
### Output Text:
The average age of patients was 35 . 6 + / - 28 . 3 years , and 62 % were males . 