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: Adverse interaction between beta - adrenergic blocking drugs and verapamil - - report of three cases .
### Output Text:
Adverse interaction between <span class="chemical">beta - adrenergic blocking drugs</span> and <span class="chemical">verapamil</span> - - report of three cases . 