#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 calcularePos(int pos,int last,float aux)
{
    if(pos==k)
    {
        sum+=aux;
        ++coun;
        return;
    }
    FOR(i,last+1,n)
    {
       calcularePos(pos+1,i,aux*x[i-1]);
    }
}
void reading()
{
    f >> n >> k;
    x.RS(n);
    FORN(i, n)
    {
        f >> x[i];
    }
    f.close();
}
void solving()
{
    calcularePos(0,0,1);
    float au = sum / coun;
    g << au;
}
void write_data()
{
}
int main()
{
    reading();
    solving();
    write_data();
}