#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <vector>

using namespace std;

#define file_in "dezastru.in"
#define file_out "dezastru.out"

#define nmax 30

int n,k;
double p[nmax];
double c[nmax][nmax];
double a[nmax][nmax];

void citire()
{
    freopen(file_in,"r",stdin);
    freopen(file_out,"w",stdout);

    scanf("%d %d", &n, &k);
    for (int i=1;i<=n;++i)
         scanf("%lf", &p[i]);
}

void solve()
{
   for (int i=0;i<=n;++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];

    c[0][0]=1;
    for (int i=1;i<=n;++i)
    {
         c[i][0]=1;
         for (int j=1;j<=k;++j)
              c[i][j]=c[i-1][j]+c[i-1][j-1];
    }

    printf("%lf\n", a[n][k]/c[n][k]);
}

int main()
{
    citire();
    solve();

    fclose(stdin);
    fclose(stdout);

    return 0;
}