Physarum can compute shortest paths: A short proof

The purpose of this note is to give a short proof that a standard model for the Physarum polycephalum slime mold correctly computes the shortest path in an undirected weighted graph [V. Bonifaci, K. Mehlhorn, G. Varma, Physarum can compute shortest paths, in Proc. of the 23rd ACM-SIAM Symposium on Discrete Algorithms, pp. 233-240, SIAM 2012].