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 Julia (1.4.0) (judge verdict: Accepted)

readInt() = parse(Int,readline())

sigma(a,n) = div((n+1)n*a,2)

function solve()
    ans = 0
    N = readInt()
    f = fill(0, N)
    for i in 1:N
        ans += sigma(i,div(N,i))
    end
    println(ans)
end

solve()