Whenever you create a .adb file which represents a program (opposed to a package), that program must be ONE procedure. Remember that you can declare procedures and functions within a declarative area, so you don't have to write everything within one single block (luckily!) So the program goes something like this:
If you are a C/C++ programmer, this is quite different than the usual for you. (Although newer version of C++ will soon be capable of sub-declaration of functions without having to create a class.)