#include <cstdio>
using namespace std;

const int MAXN = 30;

double p[MAXN];
double a[MAXN][MAXN];
int n,k;

void citire()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf ("%d%d",&n,&k);
    for (int i = 1;i <= n;++i)
        scanf ("%lf",&p[i]);
}

long long combinari(int n,int k)
{
    long long rasp = 1;
    if (n - k > k)
    {
        for (int i = n - k + 1;i <= n;++i)
            rasp *= i;
        for (int i = 1;i <= k;++i)
            rasp /= k;
    }
    else
    {
        for (int i = k + 1;i <= n;++i)
            rasp *= i;
        for (int i = 1;i <= n - k;++i)
            rasp /= i;
    }
    return rasp;
}

void dinamica()
{
    a[0][0] = 1;
    for (int i = 1;i <= n;++i)
        for (int j = 0;j <= i;++j)
            a[i][j] = a[i - 1][j] + a[i - 1][j - 1] * p[i];
}

int main()
{
    citire();
    dinamica();
    printf ("%.6lf",a[n][k] / combinari(n,k));
    return 0;
}