
ZOJ Problem Set  2673
A regular hexagon with side length n is divided into 6n^{2} unit triangles. Your task is to cover it with rhombic dominoes  pieces composed of two unit triangles sharing a side. Each domino must be placed in such a way, that it covers exactly two unit triangles. No triangle must be covered with more than one domino. Count the number of ways to do so. For example there are two ways to cover a hexagon with side length~1, they are shown on the picture.InputEace re several test cases in the input. Each case contains n (1 <= n <= 7 ). There is an empty line between each case.OutputOutput the number of ways to cover hexagon with rombic dominoes. There should be an empty line between each case.Example
Source: Andrew Stankevich's Contest #8 