Sum of all even numbers in range L and R

Given two integers L and R, the task is to find the sum of all even numbers in range L and R. 
Examples: 
 

Input: L = 2, R = 5 
Output: 6
2 + 4 = 6

Input: L = 3, R = 8
Output: 18

Method-1: Iterate from L to R and sum all the even numbers in that range. 
Method-2: Find the sum all the natural numbers from L to R and subtract the sum of odd natural numbers in range L to R from it.
Method-3: 
 

• Find the sum of all even numbers up to R i.e. No. of even numbers up to R will be R/2.
• Find the sum of all even numbers up to L-1 i.e. No. of even numbers up to L-1 will be (L-1)/2.
• Then subtract sumUptoL from sumuptoR.

Sum of all even numbers up to any N will be: 
R*(R+1) where R = N/2

Below is the implementation of the above approach:
 

// C++ program to print the sum
// of all even numbers in range L and R
#include <bits/stdc++.h>
using namespace std;

// Function to return the sum of
// all natural numbers
int sumNatural(int n)
{
    int sum = (n * (n + 1));
    return sum;
}

// Function to return sum
// of even numbers in range L and R
int sumEven(int l, int r)
{
    return sumNatural(r/2) - sumNatural((l-1) / 2);
}

// Driver Code
int main()
{
    int l = 2, r = 5;
    cout << "Sum of Natural numbers from L to R is "
         << sumEven(l, r);

    return 0;
}

// Java program to print the sum 
// of all even numbers in range L and R 

import java.io.*;

class GFG {
    
// Function to return the sum of 
// all natural numbers 
static int sumNatural(int n) 
{ 
    int sum = (n * (n + 1)); 
    return sum; 
} 

// Function to return sum 
// of even numbers in range L and R 
static int sumEven(int l, int r) 
{ 
    return sumNatural(r/2) - sumNatural((l-1) / 2); 
} 

// Driver Code 
    
    public static void main (String[] args) {

        int l = 2, r = 5; 
        System.out.println ("Sum of Natural numbers from L to R is "+
         sumEven(l, r)); 
    }
}

# Python 3 program to print the sum 
# of all even numbers in range L and R 

# Function to return the sum  
# of all natural numbers 
def sumNatural(n):
    sum = (n * (n + 1))
    return int(sum)

# Function to return sum 
# of even numbers in range L and R 
def sumEven(l, r):
    return (sumNatural(int(r / 2)) - 
            sumNatural(int((l - 1) / 2)))

# Driver Code 
l, r = 2, 5
print("Sum of Natural numbers", 
             "from L to R is", sumEven(l, r)) 

# This code is contributed 
# by 29AjayKumar

// C# program to print the sum 
// of all even numbers in range L and R

using System;

public class GFG{
        
// Function to return the sum of 
// all natural numbers 
static int sumNatural(int n) 
{ 
    int sum = (n * (n + 1)); 
    return sum; 
} 

// Function to return sum 
// of even numbers in range L and R 
static int sumEven(int l, int r) 
{ 
    return sumNatural(r/2) - sumNatural((l-1) / 2); 
} 

// Driver Code 
    
    static public void Main (){
            int l = 2, r = 5; 
        Console.WriteLine("Sum of Natural numbers from L to R is "+
        sumEven(l, r)); 
    }
}

<?php
// PHP program to print the sum of
// all even numbers in range L and R

// Function to return the sum of
// all natural numbers
function sumNatural($n)
{
    $sum = ($n * ($n + 1));
    return $sum;
}

// Function to return sum of
// even numbers in range L and R
function sumEven($l, $r)
{
    return sumNatural((int)($r / 2)) - 
           sumNatural((int)(($l - 1) / 2));
}

// Driver Code
$l = 2; $r = 5;
echo "Sum of Natural numbers " .
     "from L to R is " . sumEven($l, $r);

// This code is contributed 
// by Akanksha Rai
?>

<script>

// Javascript program to print the sum 
// of all even numbers in range L and R

// Function to return the sum of 
// all natural numbers 
function sumNatural(n) 
{ 
    let sum = Math.floor(n * (n + 1)); 
    return sum; 
} 

// Function to return sum 
// of even numbers in range L and R 
function sumEven(l, r) 
{ 
    return sumNatural(Math.floor(r/2)) - sumNatural(Math.floor(l-1) / 2); 
} 


// driver program
    
        let l = 2, r = 5; 
        document.write ("Sum of Natural numbers from L to R is "+
         sumEven(l, r)); 
   
</script>

Sum of Natural numbers from L to R is 6

Time Complexity: O(1), since there is no loop or recursion.
Auxiliary Space: O(1), since no extra space has been taken.