#include <fstream>
#include <iomanip>
#include <cmath>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");

int N, K;
long long cmb=1;
double a[26][26], p[26];

int main()
{
    f>>N>>K; a[0][0]=1;
    for (int i=1; i<=N; ++i)
        f>>p[i], a[i][0]=1;

    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];
    for (int i=2; i<=N; ++i) cmb*=i;
    for (int i=2; i<=K; ++i) cmb/=i;
    for (int i=2; i<=N-K; ++i) cmb/=i;

    g<<setprecision(6)<<fabs(a[N][K]/(double)cmb)<<'\n';
    return 0;
}