#include <bits/stdc++.h>
using namespace std;
using namespace std::chrono;
template <class e, class t, int N>
std::basic_istream<e, t>& operator>>(std::basic_istream<e, t>& in,
                                     const e (&literal)[N]) {
  std::array<e, N - 1> buffer;
  in >> buffer[0];
  if (N > 2) in.read(&buffer[1], N - 2);
  if (strncmp(&buffer[0], literal, N - 1))
    in.setstate(in.rdstate() | std::ios::badbit);
  return in;
}
template <class e, class t>
std::basic_istream<e, t>& operator>>(std::basic_istream<e, t>& in,
                                     const e& literal) {
  e buffer;
  in >> buffer;
  if (buffer != literal) in.setstate(in.rdstate() | std::ios::badbit);
  return in;
}
template <class e, class t, int N>
std::basic_istream<e, t>& operator>>(std::basic_istream<e, t>& in,
                                     e (&carray)[N]) {
  return std::operator>>(in, carray);
}
template <class t1, class t2>
istream& operator>>(istream& in, pair<t1, t2>& p) {
  in >> p.first >> p.second;
  return in;
}
template <class t>
istream& operator>>(istream& in, vector<t>& v) {
  for (int i = 0; i < v.size(); i++) {
    in >> v[i];
  }
  return in;
}
template <class t>
ostream& operator<<(ostream& out, vector<t>& a) {
  for (auto x : a) {
    out << x << " ";
  }
  return out;
}
template <class t1, class t2>
ostream& operator<<(ostream& out, map<t1, t2>& a) {
  for (auto x : a) {
    out << x << " ";
  }
  return out;
}
template <class t>
ostream& operator<<(ostream& out, set<t>& a) {
  for (auto x : a) {
    out << x << " ";
  }
  return out;
}
template <class t1, class t2>
ostream& operator<<(ostream& out, const pair<t1, t2>& a) {
  out << a.first << " " << a.second;
  return out;
}
template <class t1, class t2>
pair<t1, t2> operator+(const pair<t2, t1>& a, const pair<t1, t2>& b) {
  return {a.first + b.first, a.second + b.second};
}
template <class t1, class t2>
pair<t1, t2> operator-(const pair<t2, t1>& a, const pair<t1, t2>& b) {
  return {a.first - b.first, a.second - b.second};
}
long long gcd(long long a, long long b) { return b == 0 ? a : gcd(b, a % b); }
long long lcm(long long a, long long b) { return a * (b / gcd(a, b)); }
long long fpow(long long b, long long e, long long m = (long long)(1e9 + 7)) {
  long long r = 1;
  while (e) {
    if (e & 1) r = (r * b) % m;
    e >>= 1;
    b = (b * b) % m;
  }
  return r;
}
long long n, m;
vector<long long> a;
long long c[100][100];
long double dp[100][100][100];
void init() {
  for (long long i = 0; i < (long long)60; i++)
    for (long long j = 0; j < (long long)60; j++)
      for (long long k = 0; k < (long long)70; k++) dp[i][j][k] = -2;
  for (long long i = 0; i < (long long)51; i++) {
    for (long long j = 0; j < (long long)51; j++) {
      if (j == 0)
        c[i][j] = 1;
      else if (i == 0)
        c[i][j] = 0;
      else
        c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
    }
  }
}
long double solve(long long i, long long k, long long mx) {
  if (i == m) return (!k ? mx : 0);
  long double& r = dp[i][k][mx];
  if (r > -1) return r;
  long double p = 1;
  r = 0;
  for (long long j = 0; j < (long long)k + 1; j++) {
    long long len = (j + a[i] - 1) / a[i];
    r += (c[k][j] * p * solve(i + 1, k - j, max(mx, len)));
    p /= m;
  }
  return r;
}
int32_t main(int32_t argc, char const* argv[]) {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  cin >> n >> m;
  a.resize(m);
  cin >> a;
  init();
  cout << setprecision(15) << solve(0, n, 0) << "\n";
  return 0;
}
