program euler2
      real x(2),t,dt,tfinal
      integer count,i

      open(unit=13,file="data.d")
      t = 0
      tfinal = 20
      dt = 0.25
      x(1) = 1
      x(2) = 0
      count = tfinal/dt
      write(13,*) t,x(1),x(2)

      do 10 i=1,count
         x(1) = x(1) + x(2)*dt
         x(2) = x(2) - x(1)*dt
         t = t + dt
         write(13,*) t,x(1),x(2)
 10   continue

      stop 
      end