#include <fstream>
#include <iomanip>
using namespace std;
ifstream f ("dezastru.in");
ofstream g ("dezastru.out");
int n,k;
double ans,dv,t[27],v[27][27];
int main()
{
    f>>n>>k;
    for(int i=1;i<=n;++i) f>>t[i],v[i][0]=1;
    v[0][0]=1;
    for(int i=1;i<=n;++i)
    {
        for(int j=1;j<=i;++j) v[i][j]=v[i-1][j-1]*t[i]+v[i-1][j];
    }
    double d=v[n][k],p=1;
    for(int i=k+1;i<=n;++i) p*=i;
    for(int i=2;i<=n-k;++i) p/=i;
    g<<setprecision(6)<<d/p;
}