White round Mylan A on it?

It would be as irresponsible of me to tell you what kind of pill this is, as it is for you to ask what kind of pill this is - when you offer no details.

Now you can look for yourself on different websites and search on your own:
Common Imprints | Pharmer.org -- Pill Identification Resources (http://www.pharmer.org/imprints)
Pill Identifier - Drugs.com (http://www.drugs.com/pill_identification.html)

What would be responsible of you would be to call your pharmacy or your local emergency room and ask there - ask what it is and how to properly dispose of it. If you do not know, then this is not yours and not to be taken. Are you going to call the police and report this? The pill is not yours, so maybe you should consider contacting law enforcement.