program rk2prog

      real x,t,dt,f1,f2

      dt = 0.5
      x = 1
      open(unit=13,file="data.d")

      do 10 t=0,20,dt
         write(13,*) t,x,exp(-sin(t)),exp(-sin(t))-x
         f1 = -x*cos(t)
         f2 = -(x+f1*dt)*cos(t+dt)
         x = x + 0.5*(f1+f2)*dt

 10   continue

      end