Eulerian and Hamiltonian Graphs · Medium problem

A graph on vertices $1,2,3,4$ has edges $12,23,34,41,13$ (a square with one diagonal). It has an Euler trail running between $1$ and $3$ (its only two odd-degree vertices). Counting two trails as different whenever they use the edges in a different order, how many distinct Euler trails start at $1$ and end at $3$?