Every program has what is called an "entry point" - a place in the code that the system "knows" will start the application.
It's kind of like a book - when you start reading it, you begin at the first page of text: the introduction or chapter 1, depending.
For a C console app, it's a function called
main
So add this below the
area
function:
int main()
{
float r = 0.0f;
float f = area(f);
return 0;
}