program rkdumb_main_simple
      implicit none
      integer nstep,NMAX,NSTPMX,i,j,pgopen
      parameter (NMAX=50,NSTPMX=200)
      real xx(NSTPMX),y(NMAX,NSTPMX),x1,x2,vstart(1),yy(NSTPMX)
      real y_min, y_max
      common /path/xx,y
      external derivs
      write(*,*) 'Type in initial and final values of x :'
      read(*,*) x1,x2
c Be careful the max value of nstep is 199, not 200, see rkdumb.
 100  write(*,*) 'How many steps to integrate the solution ? (<= 199)'
      read(*,*) nstep
      if (nstep.gt.199) goto 100
      if (nstep.lt.0) goto 100

      write(*,*) 'Give the initial value y1(x0) :'
      read (*,*) vstart(1)

c This is a first-order ODE, so nvar = 1
      call rkdumb(vstart,1,x1,x2,nstep,derivs)
      write(*,*) 'RK routine has completed.'

c Find Max and min values of y1(x), but here we will use only y1(x).
      y_max= y(1,1)
      y_min= y(1,1)
      do j=1,nstep
        if (y(1,j).gt.y_max) y_max = y(1,j)
        if (y(1,j).lt.y_min) y_min = y(1,j)
      end do

c Copy y(1,1:nstep) to yy(1:nstep) for PGLINE plotting
      do i=1,nstep
        yy(i) = y(1,i)
      end do

c Start plotting
      if (pgopen('/xwin').le.0) stop
      call pgpap(6.0,0.75)
      call pgenv (x1,x2,y_min,y_max,0,0)
      call pgline(nstep,xx,yy)
      call pgclos

      end


      subroutine derivs(x,y,dy)
c The reason need to pass y is because y' can be explicit function of both
c x and y, like y'(x,y).
      real x,y,dy
      dy = 2*x
      end