PROGRAM kertoma
  IMPLICIT NONE
  INTEGER :: n
  DO
    WRITE (*,'(A)',ADVANCE='no') 'Anna n: '
    READ (*,*) n
    WRITE (*,*) 'n =  ', n
    IF (n < 0) EXIT
    WRITE (*,*) 'n! = ', kert(n)
  END DO
CONTAINS
  RECURSIVE FUNCTION kert(n) RESULT(kert_arvo)
    IMPLICIT NONE
    INTEGER, INTENT(IN) :: n
    INTEGER :: kert_arvo
    IF (n == 0) THEN
      kert_arvo = 1
    ELSE
      kert_arvo = n * kert(n-1)
    END IF
  END FUNCTION kert
END PROGRAM kertoma