program second
      real x(2),t,dt,tfinish,xnew(2)
      dt = 0.25
      tfinish = 20
      x(1) = 1
      x(2) = 0
      
      open(13,file="data.d")
      
      do 10 t=0,20,dt
         write(13,*) t,x(1),x(2)
         xnew(1) = x(1) + x(2)*dt - x(1)/2.0*dt**2 
         xnew(2) = x(2) - x(1)*dt - x(2)/2.0*dt**2 
         x(1) = xnew(1)
         x(2) = xnew(2)
 10   continue
      stop
      end