#include <bits/stdc++.h>
using namespace std;
struct _ {
  _() { ios_base::sync_with_stdio(0); }
} _;
template <class A, class B>
ostream &operator<<(ostream &o, pair<A, B> t) {
  o << "(" << t.first << ", " << t.second << ")";
  return o;
}
template <class T>
string tostring(T first, int len = 0, char c = '0') {
  stringstream ss;
  ss << first;
  string r = ss.str();
  if (r.length() < len) r = string(len - r.length(), c) + r;
  return r;
}
template <class T>
void PV(T a, T b, int n = 0, int w = 0, string s = " ") {
  int c = 0;
  while (a != b) {
    cout << tostring(*a++, w, ' ');
    if (a != b && (n == 0 || ++c % n))
      cout << s;
    else
      cout << "\n";
    cout.flush();
  }
}
template <class T>
inline bool chmin(T &a, T b) {
  return a > b ? a = b, 1 : 0;
}
template <class T>
inline bool chmax(T &a, T b) {
  return a < b ? a = b, 1 : 0;
}
const long long linf = 0x3f3f3f3f3f3f3f3fLL;
const int inf = 0x3f3f3f3f;
const int mod = int(1e9) + 7;
const int N = 51;
int n, m;
int a[N];
double dp[N][N][N];
double C[N][N];
void binomialTable(int n = N - 1) {
  for (int i = 0; i <= n; i++)
    for (int j = 0; j <= i; j++) {
      C[i][j] = j > 0 ? C[i - 1][j - 1] + C[i - 1][j] : 1;
    }
}
double p[N];
int main() {
  binomialTable();
  int n, m;
  cin >> m >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  p[0] = 1;
  for (int i = 1; i < N; i++) p[i] = n * p[i - 1];
  dp[0][0][0] = 1;
  for (int i = 0; i <= n; i++)
    for (int j = 0; j <= m; j++)
      for (int first = 0; first <= m; first++) {
        double &r = dp[i][j][first];
        if (r < 1e-12 || i == n) continue;
        for (int k = 0; j + k <= m; k++) {
          int nx = max(first, (k + a[i + 1] - 1) / a[i + 1]);
          dp[i + 1][j + k][nx] += r * C[m - j][k] / p[k];
        }
      }
  double res = 0;
  for (int first = 0; first <= m; first++) res += dp[n][m][first] * first;
  cout << fixed << setprecision(15) << res << endl;
  return 0;
}
