Paths on the board#1869
Consider an infinite checkered board.
We call a sequence of cells a path if every two consecutive cells in the sequence are adjacent by a side. The length of a path is the number of cells in it, not counting the starting cell.
A path is called simple if it does not contain the same cell more than once.
Fix some cell on the board. How many simple paths of a given length begin at this cell?
The first line contains an integer (0 \leq n \leq 22).
Print the number of simple paths of length starting from this cell.
Example 1