please change this fortran code so that it implements functions.

program circle_area implicit none character(len=4) :: end_word real :: radius, area do write(*,*) "Enter the radius of the circle: " read(*,*) radius if (radius <= 0) then write(*,*) "Error: Radius must be a positive number." end if area = 3.14159265358979323846 * radius**2 write(*,*) "The area of the circle is: ", area write(*,*) "Enter 'end' to stop the program: " read(*,*) end_word end do write(*,*) "Program terminated."

end program circle_area