PROGRAM PROG_REC_FUN_SUM3

IMPLICIT NONE

INTEGER :: N

CHARACTER :: Flag

DO

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

READ *, N

PRINT *, "Sum= ", REC_FUN_SUM3(N), " by a recursive FUNCTION"

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 FUNCTION REC_FUN_SUM3(N) RESULT (MySum)

INTEGER, INTENT(IN) :: N

INTEGER :: MySum

IF (N==3) THEN

MySum=3

ELSE

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

MySum=N+REC_FUN_SUM3(N-3)

ELSE

MySum=REC_FUN_SUM3(N-1)

END IF

END IF

END FUNCTION REC_FUN_SUM3

END PROG_REC_FUN_SUM3