#include <fstream>
#include <iomanip>
using namespace std;
int A[30],k,n,contor;
float val[30],val2[30000],sum;
bool isUsed[30];
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

bool isValid(int pos)
{
    if(isUsed[A[pos]])
        return false;
    return true;
}
void tipar()
{
    float tmp=1.0f;
    for(int i = 1;i<=k;i++)
        tmp*=val[A[i]];
    contor++;
    val2[contor]=tmp;
}

void backt(int pos)
{
    for(int x = 1;x<=n;x++)
    {
        A[pos]=x;
        if(isValid(pos))
        {
            isUsed[x] = true;
            if(pos==k){
                isUsed[A[k]]=false;
                tipar();
            }
            else
                backt(pos+1);
        }
    }
}
int main()
{
    fin>>n>>k;
    for(int i = 1;i<=n;i++)
        fin>>val[i];
    backt(1);
    for(int i = 1;i<=contor;i++)
    {
        sum+=val2[i]/contor;
    }
    fout<<setprecision(6)<<fixed<<sum;

    fout.close();
    return 0;
}