#include<fstream>
#include<iomanip>
using namespace std;
long long C[30][30];
float a[30][30],v[50];
int n,k,i,j;
int main()
{
    fstream f("dezastru.in",ios::in);
    fstream g("dezastru.out",ios::out);
    f>>n>>k;
    for (i=1;i<=n;++i) f>>v[i];
    for (i=0;i<=n;++i) a[i][0]=1;
    C[0][0]=1;C[1][1]=1;C[1][0]=1;
    for (i=1;i<=n;++i)
        for (j=1;j<=k;++j)
        {
            a[i][j]=(a[i-1][j]+a[i-1][j-1]*v[i]);
            C[i][j]=C[i-1][j]+C[i-1][j-1];
        }
    double x=a[n][k]/(double)C[n][k];
    g<<x<<setprecision(6)<<fixed;
    return 0;
}