We propose a new and faster (linear instead of exponential) way to compute the acceptability probability of an argument (in uncontroversial acyclic graph) with the grounded semantics in the constellations approach of probabilistic argumentation frameworks. Instead of computing all the worlds of the constellation (which is exponential) we show that it is possible to compute the probability of an argument only according to the acceptability probability of its direct attackers and the probability of its attacks by using a function.