$J1,\ J2,\ J3$ are rotational joints and there are two translational joints $J4$ and $J5$ between two sliders and the cross hatched rail. Sliders are free to move horizontally towards or away from one another. Now when I imagine a motion in which a force is applied vertically at $J1$, pushing or pulling on the said joint, thus sliders move away or come closer, I think it is a one degree of freedom mechanism. However the famous formula $3n-2j-3$ where $n$ is the number of links and $j$ is the number of lower pair joints, gives two degrees of freedom. Considering the rail as a ground link, with two sliders and two arms ($J1J2$ and $J1J3$) we have $n=5$ and we have $j=5$
So, what is with that?