! Name: Nicholas Farkash
  ! Purpose: To write a program that takes user input, runs it through a
  !          calculation, and outputs the results.
  ! Date: Due September 14, 2021

program HW_3
  implicit none

  real, parameter :: RH = 1.09677583e7    ! Rydberg Constant
  integer :: n1                              ! Quantum Number 1
  integer :: n2                              ! Quantum Number 2

  ! Note: n1  must be greater than n2.

  real :: frac_sub                        ! For easier calculations 
  real :: wavelength_m                    ! Wavelength in meters
  real :: wavelength_a                    ! Wavelength in Angstroms
  
  write(*,*) "Enter two integers for quantum numbers n1 and n2, respectively."
  write(*,*) "Note: n1 must be greater than n2."
  ! These statements direct the user to enter integers.
  
  read(*,*) n1,n2
  ! Assign user input to variables.

  frac_sub = (1.0 / n2**2) - (1.0 / n1**2)
  wavelength_m =  1.0 / (RH * frac_sub)
  ! Calculate the wavelength in meters given the user-input variables.

  wavelength_a = wavelength_m * 1.0e10
  ! Convert the wavelength in meters to Angstroms.
  
  write(*,*)"The wavelength for those quantum numbers is",wavelength_m,"meters."

  write(*,*)"This is equivalent to",wavelength_a,"Angstroms."
  
  stop 0
end program HW_3