#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

double rez;
double p[28];
int N, K;
int combs;
int st[28];
double st2[28];

inline void bkt() {
  int l = 1;
  st2[0] = 1.0;
  st[1] = -1;
  while (l > 0) {
    if (l == K) {
      for (int i = st[l] + 1; i < N; ++i)
        rez += st[l - 1] * p[i], ++combs;
      --l;
      continue;
    }
    ++st[l];
    if (st[l] == N)
      --l;
    else {
      st2[l] = st2[l - 1] * p[st[l]];
      ++l;
      st[l] = st[l - 1];
    }
  }
}

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();
  printf("%.6lf\n", rez / combs);
  return 0;
}