Complexity of finding the shortest simple even s-t-path Consider a graph $G=(V,E)$ and two vertices $s,t$.

What is the complexity of finding the length of the shortest simple $s-t$ path that has even length?
Does the problem become harder if the graph is edge weighted?


Note that the requirement that the path is simple makes it harder than to simply to use two copies of the graph
Its in P for undirected graphs and NP hard for directed once:
http://onlinelibrary.wiley.com/doi/10.1002/net.3230140403/pdf