#include <fstream>
#include <cstring>
#include <iomanip>

using namespace std;

ifstream in("dezastru.in");
ofstream out("dezastru.out");

const int NMAX = 25;

int Ans = 0;
double Sum = 0;
double v[NMAX+2];
int N,K, viz[NMAX+2];
double Prod = 1;

void BKT( int pas ) {
    if( pas == K+1 ) {
        ++Ans;
        Sum += Prod;
    }
    double Cprod = Prod;
    for( int i = 1;  i <= N;  ++i ) {
        if( !viz[i] ) {
            Prod *= v[i];
            viz[i] = 1;
            BKT( pas+1 );
            viz[i] = 0;
            Prod = Cprod;
        }
    }
}

int main() {
    in >> N >> K;
    for( int i = 1;  i <= N;  ++i )  in >> v[i];
    BKT(1);
    out << setprecision(6) << fixed << Sum/Ans << '\n';
    return 0;
}