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 diagnosis of P and N - induced skin reactions was based in vivo challenge .
### Output Text:
The diagnosis of <span class="chemical">P</span> and <span class="chemical">N</span> - induced skin reactions was based in vivo challenge . 