DATA: output TYPE i.

PARAMETERS: p_num TYPE i.

START-OF-SELECTION.
  PERFORM factorial USING p_num
                    CHANGING output.
  WRITE:/ output.

*&---------------------------------------------------------------------*
*&      Form  factorial
*&---------------------------------------------------------------------*
*       text
*----------------------------------------------------------------------*
*  -->  p1        text
*  <--  p2        text
*----------------------------------------------------------------------*
FORM factorial USING num
               CHANGING out.
  DATA: lv_num1 TYPE i,
        out1 TYPE i.
  IF num > 1.
    lv_num1 = num - 1.
    PERFORM factorial USING lv_num1
                      CHANGING out1.
    out = num * out1.
  ELSE.
    out = num.
  ENDIF.
ENDFORM.                    " factorial