!
! This program written by Clint Whaley for CS 1073.  It is an optional
! programming exercise given on 9/29/09.
!
PROGRAM series
!
!  This program prints out the Nth entry for the series:
!    S(i) = 100*S(i-2) + S(i-1)
!  The user is prompted for N and the seed values S(0) and S(1)
!
   IMPLICIT NONE
   INTEGER :: i, N
   DOUBLE PRECISION :: S0, S1, SN
!
!  Prompt user for series number and seed values.
!
   PRINT *, 'Please enter the sequence value (>=0) you wish to compute:'
   READ *, N
   PRINT *, 'Please enter S0'
   READ *, S0
   PRINT *, 'Please enter S1'
   READ *, S1
!
!  Do some basic error checking on numbers
!
   IF (N .LT. 0) THEN
      PRINT *, 'Negative N not allowed!'
      STOP
   ELSE IF (N .EQ. 0) THEN
      PRINT *, S0
      STOP
   ELSE IF (N .EQ. 1) THEN
      PRINT *, S1
      STOP
!
!  As an optimization, don't use a loop if both answers are 0: all #s will be 0!
!
   ELSE IF (S0.EQ.0.0 .AND. S1 .EQ.0) THEN
      PRINT *, 0.0
      STOP
   END IF
!
!  Find the Nth entry in the sequence
!
   DO i = 2, N
      SN = 100.0*S0 + S1
      S0 = S1
      S1 = SN
   END DO

   PRINT *, 'The ', N, 'th number in the sequence is:', SN

END PROGRAM series