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							Sum of All Even Numbers
								Certification: Basic Level
								Accuracy: 83.33%
								Submissions: 12
								Points: 5
							
							Write a C++ program to compute the sum of even numbers in the range [1, n].
Example 1
- Input: n = 10
 - Output: 30
 - Explanation: 
- Step 1: Identify the even numbers in the range [1, 10]: 2, 4, 6, 8, 10
 - Step 2: Calculate the sum of these even numbers: 2 + 4 + 6 + 8 + 10 = 30
 - Step 3: Return the sum: 30
 
 
Example 2
- Input: n = 15
 - Output: 56
 - Explanation: 
- Step 1: Identify the even numbers in the range [1, 15]: 2, 4, 6, 8, 10, 12, 14
 - Step 2: Calculate the sum of these even numbers: 2 + 4 + 6 + 8 + 10 + 12 + 14 = 56
 - Step 3: Return the sum: 56
 
 
Constraints
- 1 ≤ n ≤ 10^6
 - Note: You can optimize using the formula: sum of first k even numbers = k(k+1)
 - Time Complexity: O(1) when using the formula, O(n) for iterative approach
 - Space Complexity: O(1)
 
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Solution Hints
- Use arithmetic series formula: k * (k + 1) where k = n//2
 - Iterate with step size 2 for optimization
 - Return 0 if n < 2