#include <iostream>
#include <fstream>
using namespace std;
ifstream si("dezastru.in");
ofstream so("dezastru.out");
double v[30],s;
int n,c,nr;
inline void backt(int k,int x,double p)
{
    if(k==c)
    {
        s+=p;
        nr++;
        return;
    }
    for(int i=x;i<=n-c+k+1;++i)
        backt(k+1,i+1,p*v[i]);
}
int main()
{
    si>>n>>c;
    for(int i=1;i<=n;i++)
        si>>v[i];
    backt(0,1,1);
    so<<s/nr;
    return 0;
}