PROGRAM PROG_REC_SUB_SUM3

IMPLICIT NONE

INTEGER :: N, Z

CHARACTER :: Flag

DO

PRINT *, "Enter a positive integer (N), greater than or equal to 3:"

READ *, N

CALL REC_SUB_SUM3(N,Z)

PRINT *, "Sum= ", Z, " by a recursive SUBROUTINE"

PRINT *, 'Continue? If not, press "n" or "N"; else press any other key.'

READ *, Flag

IF ((Flag=="n") .OR. (Flag=="N")) THEN

EXIT

END IF

END DO

CONTAINS

RECURSIVE SUBROUTINE REC_SUB_SUM3(N, MySum)

INTEGER, INTENT(IN) :: N

INTEGER, INTENT(OUT) :: MySum

IF (N==3) THEN

MySum=3

ELSE

IF (MOD(N,3)==0) THEN

CALL REC_SUB_SUM3(N-3, MySum)

MySum=N+MySum

ELSE

CALL REC_SUB_SUM3(N-1, MySum)

END IF

END IF

END SUBROUTINE REC_SUB_SUM3

END PROG_REC_SUB_SUM3