*********************************************************************
*                                                                   *
*       takes pseudo-derivative                                     *
*                                                                   *
*********************************************************************

	SUBROUTINE DERIVE
	  INCLUDE "rpavar.inc"
	  REAL DERIV(2,100),DIST

	  DO 300 I=1,N-1
	     DIST=VOLT(I+1)-VOLT(I)
  300	     DERIV(1,I)=-(DATA(I+1)-DATA(I))/DIST
	  DERIV(1,N)=0.

	  DERIV(2,1)=0.
	  DO 310 I=2,N-1
	     DIST=VOLT(I+1)-VOLT(I-1)
  310	     DERIV(2,I)=-(DATA(I+1)-DATA(I-1))/DIST
 	  DERIV(2,N)=0.

	  OPEN(2,FILE='aaa.dat')
	  DO 320 I=1,N
  320        WRITE(2,*) VOLT(I),DATA(I),DERIV(1,I),DERIV(2,I)
	  CLOSE(2)

	  RETURN
	  END