In a five-sided figure, the vertices are labeled $X,Y,Z,W,$ and $V$. The numbers $10,12,14,16,$ and $18$ are assigned to these vertices, although not necessarily in this order. The sums of the numbers at the ends of the line segments $XY,YZ,ZW,WV,$ and $VX$ form an arithmetic sequence, although not necessarily in this order. What is the middle term of the sequence?