The point P is the intersection of the straight line joining the points $\mathrm{Q}(2,3,5)$ and $\mathrm{R}(1,-1,4)$ with the plane $5 x-4 y-z=1$. If $S$ is the foot of the perpendicular drawn from the point $T(2,1,4)$ to $Q R$, then the length of the line segment PS is