#include <iostream>
#include <fstream>
using namespace std;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

unsigned long long i,n,k,st[30],f[30],c;
double v[30],p=1.0f,rez;

void back(int niv)
{
    int i;
    for(i=st[niv-1]+1;i<=n;i++)
    {
        st[niv]=i;
        p*=v[i];
        if(niv==k)
            rez+=p,c++;
        else back(niv+1);
        p/=v[i];
    }
}
int main()
{
    fin>>n>>k;
    for(i=1;i<=n;i++)
        fin>>v[i];
    back(1);
    fout<<rez/c;
    return 0;
}