#include<stdio.h>

#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 N, K;
double rez = 0;
double cur = 1.0;
int st[MAXN];

void bkt(int l) {
  double bak_cur;
  if (l == K) {
    rez += cur;
    return;
  }
  for (int i = st[l] + 1; i + (K - l - 1) <= N; ++i) {
    bak_cur = cur;
    cur *= p[i - 1];
    st[l + 1] = i;
    bkt(l + 1);
    cur = bak_cur;
  }
}

int main() {
  freopen(InFile, "r", stdin);
  freopen(OuFile, "w", stdout);
  scanf("%d%d", &N, &K);
  for (int i = 0; i < N; ++i)
    scanf("%lf", &p[i]);
  bkt(0);
  int comb = 1;
  for (int i = 1; i <= K; ++i) {
    comb *= N - K + i;
    comb /= i;
  }
  rez /= comb;
  printf("%.6lf\n", rez);
  return 0;
}