#include <bits/stdc++.h>
using namespace std;
const int N = 60;
double pr[N][N][N];
long long C[N][N];
string to_string(string s) { return '"' + s + '"'; }
string to_string(const char* s) { return to_string((string)s); }
string to_string(bool b) { return (b ? "true" : "false"); }
template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto& x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  vector<int> a(m);
  for (int i = 0; i < m; i++) {
    cin >> a[i];
  }
  for (int i = 0; i < N; i++) {
    C[i][0] = 1;
    C[i][i] = 1;
  }
  for (int i = 1; i < N; i++) {
    for (int j = 1; j < i; j++) {
      C[i][j] += C[i - 1][j] + C[i - 1][j - 1];
    }
  }
  pr[0][0][0] = 1;
  auto Power = [&](double x, int e) {
    double ret = 1;
    for (int i = 0; i < e; i++) {
      ret *= x;
    }
    return ret;
  };
  auto Binomial = [&](int n, int k, double pr) {
    return C[n][k] * Power(pr, k) * Power(1 - pr, n - k);
  };
  for (int i = 0; i < m; i++) {
    for (int j = 0; j <= n; j++) {
      for (int k = 0; k <= n; k++) {
        for (int p = 0; k + p <= n; p++) {
          int q = (p + a[i] - 1) / a[i];
          int mx_q = max(j, q);
          pr[i + 1][mx_q][k + p] += pr[i][j][k] * C[n - k][p];
        }
      }
    }
  }
  double ans = 0;
  for (int j = 0; j <= n; j++) {
    ans += j * pr[m][j][n];
  }
  ans /= Power(m, n);
  cout << setprecision(10) << fixed << ans << '\n';
  return 0;
}
