Change blog’s index.php to blog.php???

I am trying to install wordpress into the root directory of my existing website.. It installs great! Works great! The problem is I already have an index.php in my root directory and wordpress seems to need to be named index.php.. I had to delete my existing index.php to have wordpress’s index.php located in my root directory..

How can I still use my existing index.php while also having wordpress installed in that same directory??