program sho
	call initial(x0,v,w2,t,dt,dt2)
	t = 0
	x = x0
	do  i = 1,1000
	call output(x,v,t)
	call update(x,v,w2,t,dt,dt2)
        end do
        stop
	end
*
	subroutine initial(x0,v,w2,t,dt,dt2)
	write (*,*) 'pocetna pozicija u metrima x0 =' 
	read (*,*) x0
*     pocetna brzina (m/s)
      v = 0.
*	ugaona frekvencija 
      write (*,*) 'odnos k/m =' 
	read (*,*) w2 
	write (*,*) 'vremenski korak dt =' 
	read (*,*) dt
	dt2 = 0.5 * dt
	write (8,15)
15    format(3x,'vreme',10x,'koordinata',10x,'brzina')
	return
	end
*
      subroutine update(x,v,w2,t,dt,dt2)
	a = - w2 * x
	vmid = v + a * dt2
	xmid = x + v * dt2
	amid = - w2 * xmid
	v = v + amid * dt
	x = x + vmid * dt
	t = t + dt
	return
	end 
*	
	subroutine output(x,v,t)
	write (8,*) t,x,v
      return
	end