/*
dezastru infoarena
*/
#include<vector>
#include<string>
#include<string.h>
#include<algorithm>
#include<cstdio>
#include<fstream>
#include<iostream>
#include<ctime>
#include<set>
#include<map>
#include<cmath>

using namespace std;

#define LL long long
#define PII pair<int ,int>
#define PCI pair<char ,int>
#define VB vector <bool>
#define VI vector <int>
#define VC vector <char>
#define WI vector<VI>
#define WC vector<VC>
#define RS resize
#define X first
#define Y second

#define FORN(i,n) for(int i=0;i<n;++i)
#define FOR(i,a,b) for(int i=a;i<=b;++i)
#define FORD(i,a,b) for(int i=a;i>=b;--i)
#define REPEAT do
#define UNTIL(x) while((x))

#define IN_FILE "dezastru.in"
#define OUT_FILE "dezastru.out"
ifstream f(IN_FILE);
ofstream g(OUT_FILE);

//variables
int n, k,coun;
vector<float> x;
float sum;
//other functions
void generate(VI elem, int lenght,float suma)
{
	if (lenght == 0)
	{
		sum += suma;
		++coun;
		return;
	}
	FOR(i, elem.back() + 1, n)
	{
		VI elem2 = elem;
		elem2.push_back(i);
		generate(elem2, lenght - 1,suma*x[i-1]);
	}
}
void reading()
{
	f >> n >> k;
	x.RS(n);
	FORN(i, n)
	{
		f >> x[i];
	}
	f.close();
}
void solving()
{
	VI aux;
	aux.push_back(0);
	generate(aux,k,1);
	float au = sum / coun;
	g << au;
}
void write_data()
{
}
int main()
{
	reading();
	solving();
	write_data();
}