#include<fstream>
#include<iomanip>

using namespace std;

const int nmax=35;

int c[nmax][nmax];
double v[nmax],d[nmax][nmax];

int main()
{
    ifstream fin("dezastru.in");
    ofstream fout("dezastru.out");
    int j,n,i,k;
    fin>>n>>k;
    for(i=1;i<=n;++i)
        fin>>v[i];
    for(i=0;i<=n;++i)
        c[i][0]=d[i][0]=1;
    for(i=1;i<=n;++i)
        for(j=1;j<=k;++j)
            c[i][j]=c[i-1][j-1]+c[i-1][j];
    for(int i=1;i<=n;++i)
        for(int j=1;j<=k;++j)
            d[i][j]=d[i-1][j]+d[i-1][j-1]*v[i];
    fout<<setprecision(6)<<(double)((d[n][k])/(c[n][k]));
}