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 C (GCC 9.2.1) (judge verdict: Time Limit Exceeded)

#include <stdio.h>
#include <string.h>

long go(int);

int main(){
    long i, j, total=0;
    scanf("%ld",&i);
    for(j=0;j<i;j++){
        total += (j+1)*go(j+1);
    }
    printf("%ld",total);
    return 0;
}

long go(int n){
    int i, s=0;
    for(i=1;i<=n;i++){
        if(n % i==0){
            s++;
        }
    }
    return s;
}