Problem:
Score : 400 points

Problem Statement

For a positive integer X, let f(X) be the number of positive divisors of X.

Given a positive integer N, find \sum_{K=1}^N K\times f(K).

Constraints

1 \leq N \leq 10^7

Input

Input is given from Standard Input in the following format:

N

Output

Print the value \sum_{K=1}^N K\times f(K).

Sample Input 1

4

Sample Output 1

23

We have f(1)=1, f(2)=2, f(3)=2, and f(4)=3, so the answer is 1\times 1 + 2\times 2 + 3\times 2 + 4\times 3 =23.

Sample Input 2

100

Sample Output 2

26879

Sample Input 3

10000000

Sample Output 3

838627288460105

Watch out for overflows.

Solution in Java (OpenJDK 11.0.6) (judge verdict: Accepted)


import java.util.*;

public class Main {

	public static void main(String[] args) {

		// 入力
		Scanner sc = new Scanner(System.in);
		long n = new Integer(Integer.parseInt(sc.next()));

		long result = 0;
		for (long i = 1; i <= n; i++) {
			long kosuu = n / i;
			result += (i + (i * kosuu)) * kosuu / 2;
		}

		System.out.println(result);

	}
}