```
#include <cstdio>
#include <iostream> 
#include <algorithm>

int main()
{
    int n;
    double sum = 0.0f;
    std::cin >> n; 
    std::pair<int, int> * a = new std::pair<int, int>[n];
    for (int i = 0; i < n; ++i)
    {
        std::cin >> a[i].first;
        a[i].second = i + 1;
    }
    
    std::sort(a, a + n);
    for (int i = 0; i < n; ++i)
    {
        sum += a[i].first * (n - i - 1);
    }
    
    printf("%.2f", sum / n);
    
    delete[] a;
    return 0; 
}
```