Sum of even numbers from M to N

Calculate the sum of all even numbers within a given range from M to N (inclusive).

Understand the Problem

Problem Statement

Two numbers M and N are passed as the input. The program must print the sum of even numbers from M to N (inclusive of M and N).

Constraints

1 ≤ M ≤ 9999999
M < N ≤ 9999999
Both M and N are positive integers
Input values will be within standard integer limits

Examples

Example 1

Input

2
11

Output

Explanation

The even numbers from 2 to 11 are 2, 4, 6, 8, 10. Their sum is 2 + 4 + 6 + 8 + 10 = 30.

Example 2

Input

189
200

Output

Explanation

The even numbers from 189 to 200 are 190, 192, 194, 196, 198, 200. Their sum is 190 + 192 + 194 + 196 + 198 + 200 = 1170.

Solution

#include <stdio.h>

int main() {
    int m, n;
    scanf("%d", &m);
    scanf("%d", &n);
    
    long long sum = 0;
    
    // Start from first even number >= m
    int start = (m % 2 == 0) ? m : m + 1;
    
    // Add all even numbers from start to n
    for (int i = start; i <= n; i += 2) {
        sum += i;
    }
    
    printf("%lld\n", sum);
    return 0;
}

Time:O((N-M)/2) ≈ O(N-M) - We only iterate through even numbers

Space:O(1) - Only using a constant amount of extra space

Approach:

1. Read M and N from standard input using scanf
2. Initialize sum as long long to handle large results
3. Find the first even number in the range (if M is odd, start from M+1)
4. Loop through even numbers only by incrementing by 2
5. Add each even number to the sum
6. Print the result using printf

Visual Explanation

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