思路：

找規律。

實現：

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 int main()
 5 {
 6     ll n;
 7     while (cin >> n)
 8     {
 9         ll cnt = 0;
10         for (int i = 2; i * i <= n; i++)
11         {
12             ll tmp = n / i;
13             cnt += (i + 1
 
 + tmp) * (tmp - i) / 2;
14             cnt += (tmp - i + 1) * i;
15         }
16         cout << (cnt << 2) << endl; 
17     }
18     return 0;
19 }