#include <bits/stdc++.h>
using namespace std;
template <typename T1, typename T2>
istream& operator>>(istream& in, pair<T1, T2>& a) {
  in >> a.first >> a.second;
  return in;
}
template <typename T1, typename T2>
ostream& operator<<(ostream& out, pair<T1, T2> a) {
  out << a.first << " " << a.second;
  return out;
}
template <typename T, typename T1>
T amax(T& a, T1 b) {
  if (b > a) a = b;
  return a;
}
template <typename T, typename T1>
T amin(T& a, T1 b) {
  if (b < a) a = b;
  return a;
}
const long long INF = 1e18;
const int32_t M = 1e9 + 7;
const int32_t MM = 998244353;
const long long N = 0;
long long a[55];
double fact[55], pw[55];
double c(long long n, long long r) { return fact[n] / fact[r] / fact[n - r]; }
long long n, m;
bool vis[55][55][55];
double dp[55][55][55];
double calc(long long id, long long rem, long long mx) {
  if (id == m) {
    if (rem == 0)
      return mx;
    else
      return 0;
  }
  if (vis[id][rem][mx]) {
    return dp[id][rem][mx];
  }
  vis[id][rem][mx] = 1;
  double ans = 0;
  for (long long i = 0; i < rem + 1; i++) {
    ans += c(rem, i) / pw[i] *
           calc(id + 1, rem - i, max(mx, (i + a[id] - 1) / a[id]));
  }
  return dp[id][rem][mx] = ans;
}
void solve() {
  cin >> n >> m;
  fact[0] = 1;
  pw[0] = 1;
  for (long long i = 1; i < 55; i++) {
    fact[i] = i * fact[i - 1];
    pw[i] = m * pw[i - 1];
  }
  for (long long i = 0; i < m; i++) {
    cin >> a[i];
  }
  cout << setprecision(10);
  cout << fixed;
  cout << calc(0, n, 0);
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  while (t--) solve();
  return 0;
}
