#include <fstream>
using namespace std;
ifstream is ("dezastru.in");
ofstream os ("dezastru.out");

int n, p, b[26];
double a[26], P, S;
bool s[26];
void Read();
void Perm(int k);

int main()
{
    Read();
    Perm(1);
    os << S;
    return 0;
}
void Read()
{
    is >> n >> p;
    for(int i = 1; i <= n; ++i)
        is >> a[i];
}
void Perm(int k)
{
    if(k > n)
    {
        P = 1;
        for(int i = 1; i <= p; ++i)
            P *= a[b[i]];
        P /= 6;
        S += P;
        return;
    }
    for(int i = 1; i <= n; ++i)
    {
        b[k] = i;
        if(!s[i])
        {
            s[i] = true;
            Perm(k+1);
            s[i] = false;
        }
    }
}