#include <fstream>
#include <iomanip>

using namespace std;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

#define DIM 26

int n, k;
double p[DIM], s[DIM][DIM], d[DIM][DIM];

void Read();
void Solve();

int main()
{
    Read();
    Solve();
    fout << fixed << setprecision(6) << s[n][k] / d[n][k];
    fin.close();
    fout.close();
    return 0;
}

void Solve()
{
    for ( int i = 0; i <= n; ++i )
        s[i][0] = d[i][0] = 1;
    for ( int i = 1; i <= n; ++i )
        for ( int j = 1; j <= k; ++j )
    {
        s[i][j] = s[i-1][j] + s[i-1][j-1] * p[i];
        d[i][j] = d[i-1][j] + d[i-1][j-1];
    }
}

void Read()
{
    fin >> n >> k;
    for ( int i = 1; i <= n; ++i )
        fin >> p[i];
}