//#include <stdio.h>
#include <fstream>
#include <iomanip>
int n, k, nc;
double p[26];
double a[26][26];
double cnk[26][26];

using namespace std;
int main()
{
    fstream fin("dezastru.in",ios::in);
    fstream fout("dezastru.out",ios::out);
    fin >> n >> k;
    for (int i=1;i<=n;i++)
    {
        fin >> p[i];
    }

    for (int i = 0; i <= 25; ++i)
    {
        a[i][0] = 1.0;
        cnk[i][0] = 1.0;
    }


    for (int i = 1; i <= n; ++i)
    {
        for (int j = 1; j <= k; ++j)
        {
            a[i][j] = a[i-1][j] + a[i-1][j-1] * p[i];
            cnk[i][j] = cnk[i-1][j] + cnk[i-1][j-1];
        }
    }

    fout << setprecision(6)<< a[n][k] / cnk[n][k];
    return 0;
}