Description

In "Minecraft", it is a common gameplay for players to build tower - shaped structures. Suppose a player wants to build a beacon tower, and the number of blocks on each layer increases according to a specific rule. One block is placed on the first layer. Starting from the second layer, the side - length of the block layer is $2$ more than that of the previous layer. Now, please write a recursive function to calculate the total number of blocks needed when building up to the nth layer.

Input Format

There is a line, and a positive integer $n$ ($1 ≤ n ≤ 20$), representing the number of layers of the tower to be built.

Output Format

There is a line, and a positive integer, which is the total number of blocks needed when building up to the nth layer.

3

35

Explanation of the sample

There is $1$ block on the first layer, $9$ blocks on the second layer, and $25$ blocks on the third layer. In total, $1 + 9 + 25 = 35$ blocks.