#include<fstream>
#include<iostream>
using namespace std;
double n,k,sol[26][26],a[26];


double combinari()
{
    double comb = 1;
    int i;
    for(i=k+1; i<=n; i++)
    {
        comb=comb*i;
    }
    for(i=2; i<=n-k; i++)
    {
        comb=comb/i;
    }
    return comb;
}
int main()
{
    ifstream f("dezastru.in");
    ofstream g("dezastru.out");
    f>>n>>k;
    int i,j;
    for(i=1; i<=n; i++)
    {
        f>>a[i];
        sol[i][0]=1.0;

    }
    sol[0][0]=1;
    for(i=1; i<=n; i++)
    {
        for(j=1; j<=i && j<=k; j++)
        {
            //cout<<i<<" "<<j<<"\n";
            sol[i][j]=sol[i-1][j]+sol[i-1][j-1]*a[i];
        }
    }

    g<<sol[(int)n][(int)k]/combinari();
    return 0;
}