#include<fstream>
using namespace std;

#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

double p[28];
int N, K;
double rez;
int combs;

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

ifstream fin(InFile);
ofstream fout(OuFile);

int main() {
  fin >> N >> K;
  for (int i = 0; i < N; ++i)
    fin >> p[i];
  bkt(0, -1, 1.0);
  fout << rez / combs;
  return 0;
}