implicit real*8 (a-h,o-z)
	F(X)=EXP(-X)-X
	OPEN(5,FILE='biseccion.dat')
	OPEN(6,FILE='biseccion.out')
	READ(5,*)XL,XU,ES,IM
  1	FORMAT(3F10.0,I5)
        
	AA=F(XL)*F(XU)

	IF(AA.GE.0.0) GOTO 310
	XR=(XL+XU)/2
	DO 240 NI=2,IM
	
	AA=F(XL)*F(XR)
	IF (ABS(AA).EQ.0.01) GOTO 300
	IF (AA.LT.0.0)XU=XR
	IF (AA.GT.0.0)XL=XR
	
	XN=(XL+XU)/2
	IF (XN.EQ.0.0) GOTO 230
	EA=ABS((XN-XR)/XN)*100
	IF (EA.LT.ES) GOTO 280
  230	XR=XN
  240   CONTINUE
	WRITE(6,2)
  2	FORMAT(' ','NO SE ENCONTRO LA RAIZ')
	WRITE(6,3)XR,EA
  3     FORMAT(' ',2F10.5)
	GOTO 310
  280 	WRITE(6,4)XN,EA,NI
  4	FORMAT(' ',2F10.5,I5)
	GOTO 310
  300	WRITE(6,5)XR
  5	FORMAT(' ','LA RAIZ EXACTA ES =',F10.5)
  310 	STOP
	END