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

using namespace std;

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

const int NMAX = 27;

double v[NMAX];
double sumf=0,c=0;
int n,k;

inline void read()
{
    in>>n>>k;
    for(int i=1;i<=n;i++)
    {
        in>>v[i];
    }
}

inline void solve(int x,double sum,int p)
{

    if(x-1==k)
    {
        sumf+=sum;
        c++;
    }
    else
    {
        for(register int i=p+1;i<=n-k+x;++i)
        {
            solve(x+1,sum*v[i],i);
        }
    }
}

int main()
{
    read();
    solve(1,1,0);
    out<<fixed<<setprecision(6)<<sumf/c;
}