#include <algorithm>
#include <bitset>
#include <cmath>
#include <fstream>
#include <iostream>
#include <queue>
#include <stack>
#include <string.h>
#include <string>
#include <vector>

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
ifstream fin ("dezastru.in");
ofstream fout ("dezastru.out");
const int INF = 0x3f3f3f3f;

const int Nmax = 30;
double p[Nmax];
int N, K;
int cnt  = 0;
double sum = 0;

void back(double prob, int lvl, int k) {
  if (k == 0) {
    sum += prob;
    ++cnt;
    return;
  }
  if (lvl < k)
    return;

  back(prob, lvl-1, k);
  back(prob*p[lvl], lvl-1, k-1);
}

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

  back(1, N, K);

  fout << (sum / cnt) << endl;

  return 0;
}