Grey Kangaroo 2017 Q25

The diagram shows a parallelogram $WXYZ$ with area $S$. The diagonals of the parallelogram meet at the point $O$. The point $M$ is on the edge $ZY$. The lines $WM$ and $ZX$ meet at $N$. The lines $MX$ and $WY$ meet at $P$. The sum of the areas of triangles $WNZ$ and $XYP$ is $\frac{1}{3}S$. What is the area of quadrilateral $MNOP$?

Input note: give your answer in terms of $S$.