Task approve or reject permission-Leave request Calendar

Question

i have created leave request workflow on Sharepoint Platform 2010. and the issue is if i raised a request it goes to my manager will get notification for approve / reject but if any other person open task list and click on that item then he can also able to approve / Reject.

 

how i restrict that his/her manager only can able to approve/ reject that item, if anyone try then it will bet error?

 

Thanks