pi is global - it's not defined within the function and it's
accessible in the whole program.area - Note there are two variables of that name! One is global
ant the other one is local inside the function ellipse_area.a - Note there are also two variables of that name. This was that catch:
Writing a = a + 3 has no point. A value is assigned to the local
variable a, but the function ends right after that, and this a is no
longer available, it will never be used.b is only local - it's an argument for the ellipse_area function.