program rkdumb_main
      implicit none
      integer nstep,NMAX,NSTPMX,n_var,i,j,pgopen
      parameter (NMAX=50,NSTPMX=200,n_var=2)
      real xx(NSTPMX),y(NMAX,NSTPMX),x1,x2,vstart(n_var),yy(NSTPMX)
      real y_min(NMAX), y_max(NMAX)
      common /path/xx,y
      external my_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(*,*) 'Your problem is an order- ',n_var,' ODE.'
      do i=1,n_var
        write(*,*) 'Give the initial value y',i,'(x0) :'
        read (*,*) vstart(i)
      end do

      call rkdumb(vstart,n_var,x1,x2,nstep,my_derivs)
      write(*,*) 'RK routine has completed.'

c Find Max and min values of yi(x), but here we will use only y1(x).
      do i=1,n_var
        y_max(i)= y(i,1)
        y_min(i)= y(i,1)
      end do
      do i=1,n_var
        do j=1,nstep
          if (y(i,j).gt.y_max(i)) y_max(i)=y(i,j)
          if (y(i,j).lt.y_min(i)) y_min(i)=y(i,j)
        end do
      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(1),y_max(1),0,0)
      call pgline(nstep,xx,yy)
      call pgclos

      end



      subroutine my_derivs(x,y,dydx)
      real x,y(2),dydx(2)
      dydx(1) = y(2)
      dydx(2) = -y(1)
      end