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: A pilot study to assess the safety of dobutamine stress echocardiography in the emergency department evaluation of cocaine - associated chest pain .
### Output Text:
A pilot study to assess the safety of <span class="chemical">dobutamine</span> stress echocardiography in the emergency department evaluation of <span class="chemical">cocaine</span> - associated chest pain . 