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# Octadecagonal Number

• Last Updated : 18 Mar, 2021

Given a number N, the task is to find the Nth Octadecagonal number.

An Octadecagonal number is a class of figurate numbers. It has 18 – sided polygon called Octadecagon. The Nth Octadecagonal number count’s the eighteen number of dots and all other dots are surrounding with a common sharing corner and make a pattern. The first few Octadecagonal numbers are 1, 18, 51, 100, 165, 246, 343 …

Examples:

Input: N = 2
Output: 18
Explanation:
The second octadecagonal number is 18.
Input: N = 6
Output: 246

Approach: The Nth Octadecagonal number is given by the formula:

Below is the implementation of the above approach:

## C++

 `// C++ program to find Nth``// Octadecagonal number` `#include ``using` `namespace` `std;` `// Function to find N-th``// Octadecagonal number``int` `Octadecagonal_num(``int` `n)``{``    ``// Formula to calculate nth``    ``// Octadecagonal number``    ``return` `(16 * n * n - 14 * n) / 2;``}` `// Driver code``int` `main()``{``    ``int` `n = 3;``    ``cout << Octadecagonal_num(n) << endl;` `    ``n = 10;``    ``cout << Octadecagonal_num(n) << endl;` `    ``return` `0;``}`

## Java

 `// Java program to find Nth``// octadecagonal number``import` `java.io.*;``import` `java.util.*;` `class` `GFG{``    ` `// Function to find N-th``// octadecagonal number``static` `int` `Octadecagonal_num(``int` `n)``{``        ` `    ``// Formula to calculate nth``    ``// octadecagonal number``    ``return` `(``16` `* n * n - ``14` `* n) / ``2``;``}``    ` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``int` `n = ``3``;``    ``System.out.println(Octadecagonal_num(n));``    ` `    ``n = ``10``;``    ``System.out.println(Octadecagonal_num(n));``}``}` `// This code is contributed by coder001`

## Python3

 `# Python3 program to find Nth``# octadecagonal number` `# Function to find N-th``# octadecagonal number``def` `Octadecagonal_num(n):``    ` `    ``# Formula to calculate nth``    ``# octadecagonal number``    ``return` `(``16` `*` `n ``*` `n ``-` `14` `*` `n) ``/` `2` `# Driver code   ``n ``=` `3``print``(``int``(Octadecagonal_num(n)))` `n ``=` `10``print``(``int``(Octadecagonal_num(n)))` `# This code is contributed by divyeshrabadiya07    `

## C#

 `// C# program to find Nth``// octadecagonal number``using` `System;` `class` `GFG{``    ` `// Function to find N-th``// octadecagonal number``static` `int` `Octadecagonal_num(``int` `n)``{``        ` `    ``// Formula to calculate nth``    ``// octadecagonal number``    ``return` `(16 * n * n - 14 * n) / 2;``}``    ` `// Driver code``public` `static` `void` `Main(``string``[] args)``{``    ``int` `n = 3;``    ``Console.Write(Octadecagonal_num(n) + ``"\n"``);``    ` `    ``n = 10;``    ``Console.Write(Octadecagonal_num(n) + ``"\n"``);``}``}` `// This code is contributed by rutvik_56`

## Javascript

 ``
Output:
```51
730```

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