auto save word document c#

I have a bit weird question but I have to get it done somehow, any help will be appreciated.

I have an application and it detects if word document is opened if it is it has to auto-save it. For example if I my program is running and I open ms office word program will detect it (done) and it has to save it (this is what I dont know). Program should be able to auto save file (save as) witho开发者_如何学Pythonut asking user for any kind of information.