real function fy(x,y)

c     Return the time-derivative of y in terms of x and y.  Modify this
c     to the one for your problem.

c     Warning: do not divide by integers; add a decimal point to make
c     them floating point numbers.

c     avoid typos
      implicit none

c declarations

c     function arguments
      real x,y

c executable

c     we have d^(theta)/d(t^2) = -sqrt(2) d^(theta)/d(t^2) - sin(theta):
      fy=0.*y-sin(x)

      return
      end