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: METHODS : We studied 9 patients with hypersensitivity to P and N with or without associated reactions to classic NSAIDs .
### Output Text:
METHODS : We studied 9 patients with hypersensitivity to <span class="chemical">P</span> and <span class="chemical">N</span> with or without associated reactions to classic NSAIDs . 