#include <bits/stdc++.h>

using namespace std;

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

double a[30],sum;
bool fr[30];
int sol[30],n,p,nrp;

void back(int k, double prod)
{
    for (int i=sol[k-1]+1; i<=n; i++ )
    {
        sol[k]=i;
        if ( k==p )
        {
            sum+=prod*a[sol[k]];
            nrp++;
        }
        else back(k+1,prod*a[sol[k]]);
    }
}

int main()
{
    f >> n >> p;
    for (int i=1; i<=n; i++ )
        f >> a[i];

    back(1,1);
    g << fixed << setprecision(6) << sum/nrp;
    return 0;
}