Do not continue until a file exists in powershell

Hi,

I was wondering if there is a method in powershell to get it to 

continually check whether a file exists and do not progress until it finds it.

i.e

DO WHILE  

c:\flag.txt NOT EXISTS

Wait 10 seconds

END

<Next command >

So that the <next Command> section is only executed once it finds the file c:\flag.txt

Is this possible?

Thanks,

Zoe