# include <fstream>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n,k,i,j;
float a[100],p[100][100],sum[100][100];
int main ()
{
    f>>n>>k;
    for (i=1; i<=n; ++i)
        f>>a[i];

    for (i=0; i<=n; ++i)
        p[i][i]=p[i][0]=sum[i][0] = 1;

    for (i=1; i<=n; ++i )
        for (j=1; j<=i; ++j )
            p[i][j] = p[i-1][j] + p[i-1][j-1] ;

    for (i=1; i<=n; ++i )
        for (j=1; j<=i; ++j )
            sum[i][j]=sum[i-1][j]+a[i]*sum[i-1][j-1];

    g<<sum[n][k]/p[n][k];

    return 0;
}