#include <fstream>
using namespace std;
ifstream f ("dezastru.in");
ofstream g ("dezastru.out");
double v[27],a[27][27],fact[27];
int n,k;
int main()
{
    f>>n>>k;
    a[0][0]=1;
    fact[0]=1;
    for (int i=1;i<=n;i++)
    {
        f>>v[i];
        fact[i]=fact[i-1]*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]*v[i];
    g<<a[n][k]/(fact[n]/fact[k]/fact[n-k]);
    return 0;
}