#include <fstream>
#include <iomanip>

using namespace std;

void Back();


int N, K;
double V[26], C[26][26], dp[26][26], final;
int number;

double now;
int k, last;

int main()
{
    ifstream fin("dezastru.in");
    ofstream fout("dezastru.out");

    fin >> N >> K;
    for (int i = 1; i <= N; ++i)
        fin >> V[i];

    for (int i = 0; i <= N; ++i)
        C[i][0] = dp[i][0] = 1;
    for (int i = 1; i <= N; ++i)
        for (int j = 1; j <= K; ++j) {
            C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
            dp[i][j] = dp[i - 1][j] + V[i] * dp[i - 1][j - 1];
        }

    double final = (double)dp[N][K] / C[N][K];
    fout << fixed << setprecision(7) << final;

    fin.close();
    fout.close();
}