#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int n,k,i,j,l;
double v[100],P,p3;
unsigned long long p1,p2;

int main()
{
    fin>>n>>k;
    p1=1;
    l=1;
    p2=1;
    for(i=1;i<=n;i++)
        p1=p1*i;
    for(i=1;i<=n;i++)
        fin>>v[i];
    for(i=1;i<n;i++)
        for(j=i+1;j<=n;j++)
        P+=(v[i]*v[j]);
        for(i=k+1;i<=n;i++)
        {p2=p2*l;
        l++;}
    P=P*k*p2;
    p3=P/p1;
    fout<<setprecision(7)<<p3<<"\n";
    return 0;
}