#include <bits/stdc++.h>
using namespace std;
template <typename T>
T gcd(T a, T b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
template <typename T>
T pow(T a, T b, long long m) {
  T ans = 1;
  while (b > 0) {
    if (b % 2 == 1) ans = ((ans % m) * (a % m)) % m;
    b /= 2;
    a = ((a % m) * (a % m)) % m;
  }
  return ans % m;
}
double fac[55];
double c[55][55];
void factorial() {
  fac[0] = 1.00;
  for (int i = 1; i <= 50; i++) {
    fac[i] = fac[i - 1] * (double)i;
  }
}
int n, m;
int a[55];
double pw(double base, int exp) {
  double ans = 1.00;
  for (int i = 0; i < exp; i++) ans *= base;
  return ans;
}
double dp[55][55][55];
bool vis[55][55][55];
double solve(int id, int cnt, int maxSoFar) {
  if (id == (m)) {
    if (cnt == 0)
      return maxSoFar;
    else
      return 0.00;
  }
  if (vis[id][cnt][maxSoFar]) return dp[id][cnt][maxSoFar];
  vis[id][cnt][maxSoFar] = 1;
  double ans = 0.00;
  for (int i = 0; i <= cnt; i++) {
    ans += c[cnt][i] *
           solve(id + 1, cnt - i, max((a[id] + i - 1) / a[id], maxSoFar));
  }
  return dp[id][cnt][maxSoFar] = ans;
}
int main() {
  cin >> n >> m;
  factorial();
  for (int i = 0; i < m; i++) cin >> a[i];
  for (int i = 0; i <= 50; i++) {
    c[i][0] = 1.00;
    c[i][i] = 1.00;
  }
  for (int i = 2; i <= 50; i++) {
    for (int j = 1; j < i; j++) {
      c[i][j] = c[i - 1][j] + c[i - 1][j - 1];
    }
  }
  double ans = solve(0, n, 0);
  ans /= pw(m, n);
  cout << fixed << setprecision(12) << ans;
}
