#include <iostream>
#include <fstream>
#include <iomanip>
#include <cstdio>
#define NMax 5000
using namespace std;

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

int n, p, foo;
int st[NMax];
long double atac[NMax];
long double prob[NMax], sol;
bool fol[NMax];

void calc()
{
    float ans = 1.000000;
    for(int i = 1; i <= p; ++i)
        ans *= atac[st[i]];
    prob[++foo] = ans;
}

void back(int k)
{
    for(int i = st[k - 1] + 1; i <= n; ++i)
    {
        if(!fol[i])
        {
            fol[i] = true;
            st[k] = i;
            if(k == p) calc();
            else back(k + 1);
            fol[i] = false;
        }
    }
}

int main()
{
    f >> n >> p;
    for(int i = 1; i <= n; ++i)
        f >> atac[i];
    f.close();
    back(1);
    for(int i = 1; i <= foo; ++i)
        sol += prob[i];
    g << setprecision(6) << sol/foo <<'\n' << fixed;
    g.close();
    return 0;
}