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: PURPOSE : To evaluate the frequency , severity and preventability of warfarin - induced cerebral haemorrhages due to warfarin and warfarin - drug interactions in patients living in the county of Osterg tland , Sweden .
### Output Text:
PURPOSE : To evaluate the frequency , severity and preventability of <span class="chemical">warfarin</span> - induced cerebral haemorrhages due to <span class="chemical">warfarin</span> and <span class="chemical">warfarin</span> - drug interactions in patients living in the county of Osterg tland , Sweden . 