program particle_in_box
      integer pgopen
      write(*,*)'What is initial vx?'
      read(*,*) vx
      write(*,*)'What is initial vy?'
      read(*,*) vy

      x_range = 10.0
      y_range = 10.0

      if( pgopen('/xwin') <= 0 ) stop
      call pgenv(0.0,x_range,0.0,y_range,0,0)

      dt = 0.0001


      do i =1, 1000000
        if(x>x_range) vx = (-1)*vx
        if(x<0.0) vx = (-1)*vx
        if(y>y_range) vy = (-1)*vy
        if(y<0.0) vy = (-1)*vy
        x = x + vx * dt
        y = y + vy * dt
c       call pgpt(1,x,y,20)
        call pgcirc(x,y,0.2)
      enddo
      call pgclos
      end