program rk
      real t,dt,x(2),tfinish,f(2),x1s
      open(13,file="data.d")
      dt = 0.5/2
      tfinish = 20
      x(1) = 1
      x(2) = 0
      do 10 t=0,tfinish,dt
         write(13,*) t,x(1),x(2)
         f(1) = f1(x(1),x(2),t)
         f(2) = f2(x(1),x(2),t)
         x1s = x(1)
         x(1) = x(1) + 0.5*dt*(f(1) + f1(x(1)+f(1)*dt, 
     c        x(2)+f(2)*dt, t+dt))        
         x(2) = x(2) + 0.5*dt*(f(2) + f2(x1s+f(1)*dt, 
     c        x(2)+f(2)*dt, t+dt))
 10   continue

      stop
      end

      real function f1(x1,x2,t)
      real x1,x2,t
         f1 = x2
      return
      end

      real function f2(x1,x2,t)
      real x1,x2,t
         f2 = -x1
      return
      end