7017: ABC172 —— D - Sum of Divisors
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Description
For a positive integer $X$, let $f(X)$ be the number of positive divisors of $X$.
Given a positive integer $N$, find $\sum_{K=1}^N K\times f(K)$.
Given a positive integer $N$, find $\sum_{K=1}^N K\times f(K)$.
Input
Input is given from Standard Input in the following format:
$N$
$N$
Output
Print the value $\sum_{K=1}^N K\times f(K)$.
Constraints
$1≤N≤10^7$
Sample 1 Input
4
Sample 1 Output
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 2 Input
100
Sample 2 Output
26879
Sample 3 Input
10000000
Sample 3 Output
838627288460105