All the numbers $1,2,3,4,5,6,7,8,9$ are written in a $3\times 3$ array of squares, one number in each square, in such a way that if two numbers are consecutive then they occupy squares that share an edge. The numbers in the four corners add up to $18$. What is the sum of the numbers in the four squares that are in the middle of each side (i.e., not corners)?