Call the previous page on the current page?

 Hi Friends,
I have a login page with a register button. There are other two pages(x and y) and on the other two pages there is a login button which direct the user to the login page. I need some code on the login page and the register button which will tell from which of the two pages(x and y) the login was called.
 
Thanks in Advance
 Sushim
 
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