I should implement this summation in C ++. I have tried with this code, but with very high numbers up to 10 ^ 12 it takes too long.

The summation is:

For any positive integer k, let d(k) denote the number of positive divisors of k (including 1 and k itself). For example, for the number 4: 1 has 1 divisor, 2 has two divisors, 3 has two divisors, and 4 has three divisors. So the result would be 8.

This is my code:

```
#include <iostream>
#include <algorithm>
using namespace std;
int findDivisors(long long n)
{
int c=0;
for(int j=1;j*j<=n;j++)
{
if(n%j==0)
{
c++;
if(j!=(n/j))
{
c++;
}
}
}
return c;
}
long long compute(long long n)
{
long long sum=0;
for(int i=1; i<=n; i++)
{
sum += (findDivisors(i));
}
return sum;
}
int main()
{
int n, divisors;
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
cin >> n;
cout << compute(n);
}
```

I think it's not just a simple optimization problem, but maybe I should change the algorithm entirely. Would anyone have any ideas to speed it up? Thank you.