#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 N, K;
double rez;
int combs;

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

int main() {
  assert(freopen(InFile, "r", stdin));
  assert(freopen(OuFile, "w", stdout));
  scanf("%d%d", &N, &K);
  for (int i = 0; i < N; ++i)
    scanf("%lf", &p[i]);
  bkt(0, -1, 1.0);
  printf("%.6lf\n", rez / combs);
  return 0;
}