#include <iostream>
#include <fstream>
#include <cstring>
using namespace std;
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");
int n,m,k,stiva[26],fact=1;
float pb,p[26];
int factorial(int k)
{
    if(k<=n) {fact=fact*k;factorial(k+1);}
    else return fact;
}
void bt(int k)
{
    for(int i=stiva[k-1]+1;i<=n;i++)
    {
        stiva[k]=i;
        if(k==m)
        {
            //for(int j=1;j<=m;j++) fout<<p[stiva[j]]<<" ";
            float c=1;
            for(int j=1;j<=m;j++) c=c*p[stiva[j]];
            pb=pb+2*c;
            //fout<<c/fact<<'\n';
        }
        else bt(k+1);
    }
}
int main()
{
    fin>>n>>m;
    for(int i=1;i<=n;i++) fin>>p[i];
    bt(1);
    fout<<pb/factorial(1);

    return 0;
}