A vector is defined as
$$f=y\hat{i}+x\hat{j}+z\hat{k}$$
where $\hat{i},\hat{j},\hat{k}$ are unit vectors in Cartesian $(x,y,z)$ coordinate system.
The surface integral 
$f.ds$ over the closed surface S of a cube with vertices having the following coordinates:
$(0,0,0), (1,0,0),(1,1,0),(0,1,0),(0,0,1),(1,0,1),(1,1,1),(0,1,1)$ is __________.