The main program unit begins with the keyword program (optional), followed by the program name (also otional); it ends with the keyword end (required), followed by program Name_Prog (optional).
program <name> declarations executable code end [ program <name> ]
Open a file hello.f90. Type the following ``hello world'' program:
! my first Fortran program program hello print*, 'Hello World!' end program hello
Compile it with f90 hello.f90. Run the resulting code with a.out.