Given a number n, find the nth Centered Octadecagonal number.

The Centered Octadecagonal Number represents a dot in the centre and others dot are arranged around it in successive layers of octadecagon(18 sided polygon). Examples :

```Input :  2
Output : 19

Input : 6
Output : 271
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

In mathematics, Centered Octadecagonal number for n-th term is given by:

``` ```

Below is the basic implementation of the above idea:

## C++

 `// C++ Program to find the ` `// nth centered octadecagonal ` `// number ` `#include ` `using` `namespace` `std; ` ` `  `// centered octadecagon  function ` `int` `center_octadecagon_num(``long` `int` `n) ` `{ ` `    ``// Formula to calculate nth ` `    ``// centered octadecagonal number ` `    ``return` `9 * n * n - 9 * n + 1; ` `} ` ` `  `// Driver Code ` `int` `main() ` `{ ` `    ``long` `int` `n = 3; ` `    ``cout << n << ``"th centered octadecagonal number : "` `                  ``<< center_octadecagon_num(n); ` `    ``cout << endl; ` `    ``n = 13; ` `    ``cout << n << ``"th centered octadecagonal number : "` `                 ``<< center_octadecagon_num(n); ` ` `  `    ``return` `0; ` `} `

## Java

 `// Java Program to find the ` `// nth centered octadecagonal ` `// number ` `import` `java.io.*; ` ` `  `class` `GFG  ` `{ ` `     `  `// centered octadecagon function ` `static` `int` `center_octadecagon_num(``int` `n) ` `{ ` `         `  `    ``// Formula to calculate nth ` `    ``// centered octadecagonal number ` `    ``return` `9` `* n * n - ``9` `* n + ``1``; ` `} ` ` `  `// Driver Code ` `public` `static` `void` `main (String[] args)  ` `{ ` `int` `n = ``3``; ` `System.out.print(n + ``"th centered "` `+  ` `           ``"octadecagonal number : "``); ` `System.out.println(center_octadecagon_num(n)); ` ` `  `n = ``13``; ` `System.out.print(n + ``"th centered "` `+  ` `           ``"octadecagonal number : "``); ` `System.out.println(center_octadecagon_num(n)); ` `} ` `} ` ` `  `// This code is contributed by ajit `

## Python3

 `# Program to find nth ` `# centered octadecagonal number ` ` `  `# Centered octadecagonal ` `# number function ` `def` `center_octadecagon_num(n) : ` `     `  `    ``# Formula to calculate  ` `    ``# nth centered octadecagonal ` `    ``# number & return it ` `    ``# into main function. ` `    ``return``(``9` `*` `n ``*` `n ``-`  `           ``9` `*` `n ``+` `1``) ` ` `  `# Driver Code ` `if` `__name__ ``=``=` `'__main__'` `: ` `         `  `    ``n ``=` `3` `    ``print``(n,``"rd centered octadecagonal "` `+` `                                ``"number : "``,  ` `                ``center_octadecagon_num(n)) ` ` `  `    ``n ``=` `13` `    ``print``(n,``"th centered octadecagonal "` `+` `                              ``"number : "``,  ` `                ``center_octadecagon_num(n)) ` `             `  `# This code is contributed ` `# by akt_mit `

## C#

 `// C# Program to find the ` `// nth centered octadecagonal ` `// number ` `using` `System; ` ` `  `class` `GFG ` `{ ` `     `  `// centered octadecagon function ` `static` `int` `center_octadecagon_num(``int` `n) ` `{ ` `         `  `    ``// Formula to calculate nth ` `    ``// centered octadecagonal number ` `    ``return` `9 * n * n - 9 * n + 1; ` `} ` ` `  `// Driver Code ` `static` `public` `void` `Main () ` `{ ` `     `  `    ``int` `n = 3; ` `    ``Console.Write( n + ``"th centered "` `+  ` `            ``"octadecagonal number : "``); ` `    ``Console.WriteLine( center_octadecagon_num(n)); ` ` `  `    ``n = 13; ` `    ``Console.Write( n + ``"th centered "` `+  ` `            ``"octadecagonal number : "``); ` `    ``Console.WriteLine(center_octadecagon_num(n)); ` `} ` `} ` ` `  `// This code is contributed by aj_36. `

## PHP

 ` `

Output :

```3th centered octadecagonal number : 55
13th centered octadecagonal numbe : 1405
```

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