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 Go (1.14.1) (judge verdict: Accepted)

package main
import "fmt"

var N int

func main(){
  fmt.Scanf("%d", &N)
  v := make([]int, N+1)
  for i:=1; i<=N; i++ {
    for j:=i; j<=N; j+=i {
    	v[j]++
    } 
  }
  ans := int64(0)
  for i:=1; i<=N; i++ {
    ans += int64(i)*int64(v[i])
  }
  fmt.Println(ans)
}