#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream f ("dezastru.in");
ofstream g ("dezastru.out");

float a[30][30], t[30], S;
int n, k;

int main()
{
    int i, j;

    f >> n >> k;

    for (i=1; i<=n; i++)
        f >> t[i];

    for (i=1; i<=n; i++)
        for (j=1; j<=n; j++)
            if (i!=j)
                a[i][j] = (t[i]*t[j])/6;

    for (i=1; i<=n; i++)
        for (j=1; j<=n; j++)
            S+=a[i][j];

    g << fixed << setprecision(6) <<  S;
}