#include<cstdio>

#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 bkt(int l, int lastidx, double cur) {
  if (l == K)
    return cur;
  double s = 0;
  for (int i = lastidx + 1, lim = N + 1 - (K - l); i <= lim; ++i)
    s += bkt(l + 1, i, cur * p[i]);
  return s;
}

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]);
  double rez = bkt(0, -1, 1.0);
  int comb = 1;
  for (int i = 1; i <= K; ++i) {
    comb *= N - K + i;
    comb /= i;
  }
  rez /= comb;
  printf("%.6lf\n", rez);
  return 0;
}