#include<iostream>
#include<fstream>
#include <iomanip>
#include <string>
#include <sstream>
using namespace std;

int N,K;
double numere[30];
double sume[30][30];
int cmb[30][30];
//int poz;
//stack<double> result;
//double data[25];
//double probab;
//double prob1=1;
//double A[25][25];
/*
int combinari(int n,int k)
{
    if (k==0) return 1;
    else if (k>n) return 0;
    else return (combinari(n-1,k)+combinari(n-1,k-1));
}*/
/*
void combinationUtil(double numere[], double data[], int start, int end, int index, int k)
{
    if (index == k) {
        double prob=1;
        cout << poz << ": ";
        for (int j=0; j<k; j++) {
          prob = prob*data[j];
          cout << data[j] << " ";
        }
        cout << "\n";
        poz++;
        probab=probab+prob;
        return;
    }
    for (int i=start; i<=end && end-i+1 >= k-index; i++)
    {
        data[index] = numere[i];
        combinationUtil(numere, data, i+1, end, index+1, k);
    }
}
*/
/*
void combinationUtil(double prob, int N, int k) {
  if (k == 0) {
    probab += prob;
    ++poz;
    return;
  }
  if (N == k) {
    for(int i = N-1; i >= 0; --i)
      prob *= numere[i];
    probab += prob;
    ++poz;
    return;
  }
  combinationUtil(prob, N-1, k);
  combinationUtil(prob*numere[N-1], N-1, k-1);
}*/
int main(){
    ifstream f("dezastru.in");
    ofstream out("dezastru.out");
    f>>N>>K;

    for (int i=1; i<=N;i++) {
        f>>numere[i];
    }

    cmb[0][0]=1;
    sume[0][0]=1;
    for(int i=1; i<=N; ++i) {
        for(int j=0; j<=K; ++j)
        {
            cmb[i][j]= cmb[i-1][j]+ cmb[i-1][j-1];
            sume[i][j]= sume[i-1][j]+ sume[i-1][j-1]*numere[i];
        }
    }
    out << setprecision(6) << (sume[N][K])/(double)(cmb[N][K]);
    return 0;
}