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Count of numbers whose 0th and Nth bits are set
• Last Updated : 10 May, 2021

Given a positive integer N, the task is to count the numbers that can be represented with N bits and whose 0th and Nth bits are set.

Examples:

Input: N = 2
Output:
All possible 2-bit integers are 00, 01, 10 and 11.
Out of which only 11 has 0th and Nth bit set.
Input: N = 4
Output: 4

Approach: Out of the given N bits, only two bits need to be set i.e. the 0th and the Nth bit. So, setting these 2 bits as 1 we are left with the rest N – 2 bits every single of which can either be 0 or 1 and there are 2N – 2 ways of doing that.
Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach``#include ``using` `namespace` `std;` `// Function to return the count of n-bit``// numbers whose 0th and nth bits are set``int` `countNum(``int` `n)``{``    ``if` `(n == 1)``        ``return` `1;``    ``int` `count = ``pow``(2, n - 2);``    ``return` `count;``}` `// Driver code``int` `main()``{``    ``int` `n = 3;``    ``cout << countNum(n);` `    ``return` `0;``}`

## Java

 `// Java implementation of the approach``import` `java.io.*;` `class` `GFG``{``    ``// Function to return the count of n-bit``    ``// numbers whose 0th and nth bits are set``    ``static` `int` `countNum(``int` `n)``    ``{``        ``if` `(n == ``1``)``            ``return` `1``;``            ` `        ``int` `count = (``int``) Math.pow(``2``, n - ``2``);``        ``return` `count;``    ``}``    ` `    ``// Driver code``    ``public` `static` `void` `main (String[] args)``    ``{``        ``int` `n = ``3``;``        ``System.out.println(countNum(n));``    ``}``}` `// This code is contributed by ajit`

## Python

 `# Python3 implementation of the approach` `# Function to return the count of n-bit``# numbers whose 0th and nth bits are set``def` `countNum(n):``    ``if` `(n ``=``=` `1``):``        ``return` `1``    ``count ``=` `pow``(``2``, n ``-` `2``)``    ``return` `count` `# Driver code` `n ``=` `3``print``(countNum(n))` `# This code is contributed by mohit kumar 29`

## C#

 `// C# implementation of the approach``using` `System;` `class` `GFG``{` `    ``// Function to return the count of n-bit``    ``// numbers whose 0th and nth bits are set``    ``static` `int` `countNum(``int` `n)``    ``{``        ``if` `(n == 1)``            ``return` `1;``            ` `        ``int` `count = (``int``) Math.Pow(2, n - 2);``        ``return` `count;``    ``}``    ` `    ``// Driver code``    ``static` `public` `void` `Main ()``    ``{``        ``int` `n = 3;``        ``Console.WriteLine(countNum(n));``    ``}``}` `// This code is contributed by AnkitRai01`

## Javascript

 ``
Output:
`2`

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