#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
double suma, v[26], contor,n,inmultire,c;
int back1( int poz, int cnt)
{
    if(cnt==1)
        {
            inmultire=1;
        }
    for(int a1=poz;a1<=n;a1++)
    {
        inmultire*=v[a1];
        if(cnt<c) {back1(a1+1,cnt+1);}
        else
        {
            suma+=inmultire;
            contor++;
        }
        inmultire/=v[a1];
    }

}
int main()
{
    fin>>n>>c;
    for(int a1=1;a1<=n;a1++)
    {
        fin>>v[a1];
    }
    back1(1,1);
    fout<<suma/contor;
}