program rk2
      
      real x,t,dt,tfinish,f1
      open(13,file="data.d")
      dt = 0.3
      tfinish = 15
      x = 0

      do 10 t=0,tfinish,dt
         write(13,*) t,x,0.4*exp(-t)+0.2*sin(2*t)-0.4*cos(2*t)
         f1 = f(x,t)
         x = x + 0.5*(f1 + f(x+f1*dt,t+dt))*dt
 10   continue
      stop
      end

      real function f(x,t)
      real x,t
      f = -x + sin(2*t)
      return
      end