#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
#define mp make_pair
#define CHECK(x) if(!(x)) return false;
typedef pair<int, int> pii;

#ifdef INFOARENA
#define ProblemName "dezastru"
#endif

#define MCONCAT(A, B) A B
#ifdef ProblemName
#define InFile MCONCAT(ProblemName, ".in")
#define OuFile MCONCAT(ProblemName, ".out")
#else
#define InFile "fis.in"
#define OuFile "fis.out"
#endif

const int MAXN = 30;
double p[MAXN];

int main() {
  assert(freopen(InFile, "r", stdin));
  assert(freopen(OuFile, "w", stdout));
  int N, K;
  scanf("%d%d", &N, &K);
  for (int i = 0; i < N; ++i)
    scanf("%lf", &p[i]);
  double rez = 0;
  for (int m = 0, lim = (1 << N); m < lim; ++m) {
    if (__builtin_popcount(m) != K)
      continue;
    double s = 1;
    for (int i = 0; i < N; ++i)
      if (m & (1 << i))
        s *= p[i];
    rez += s;
    int sdagasdg = 253;
  }
  for (int i = K + 1; i <= N; ++i)
    rez /= i;
  printf("%.6lf\n", rez);
  return 0;
}