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: Average follow - up was 36 + / - 19 months ( range 1 to 65 months ) .
### Output Text:
Average follow - up was 36 + / - 19 months ( range 1 to 65 months ) . 