PROGRAM TSTFAC

C  THIS PROGRAM CALCULATES N FACTORIAL FOR A GIVEN
C  INTEGER N AND HANDLES THE INADVERTENT ENTRY OF A
C  NEGATIVE NUMBER APPROPRIATELY

      INTEGER N
      REAL FAC
      
 10   WRITE(*,*) 'ENTER THE VALUE N (CNTRL-C TO END):'
      READ(*,*) N

C  CALCULATES N FACTORIAL FOR LEGAL VALUES OF N      
      IF (N .GE. 0) THEN
         WRITE(*,*) N, '! = ',FAC(N)
         GOTO 10
C  ERROR HANDLING FOR THE CASE WHEN N LESS THAN 0 IS ENTERED
      ELSE
         WRITE(*,*)
         WRITE(*,*) '*********************************************'
         WRITE(*,*) '*                                           *'
         WRITE(*,*) '*         INVALID VALUE FOR N               *'
         WRITE(*,*) '*  PLEASE ENTER A VALUE FOR N THAT IS >= 0  *'
         WRITE(*,*) '*                                           *'
         WRITE(*,*) '*********************************************'
         WRITE(*,*)
         GOTO 10
      ENDIF
      
      STOP
      END