# poj1286

Description

Beads of red, blue or green colors are connected together into a circular necklace of n beads ( n < 24 ). If the repetitions that are produced by rotation around the center of the circular necklace or reflection to the axis of symmetry are all neglected, how many different forms of the necklace are there?

Input

The input has several lines, and each line contains the input data n.
-1 denotes the end of the input file.
Output

The output should contain the output data: Number of different forms, in each line correspondent to the input data.
Sample Input

4
5
-1
Sample Output

21
39
Source

Xi’an 2002

n为偶数情况 考虑分割时候恰好切在两点上 循环节个数$\frac{(n-2)}{2}+2$