#include <bits/stdc++.h>
using namespace std;
inline bool ispow2(int x) { return (x != 0 && (x & (x - 1)) == 0); }
template <class T>
T __sqr(const T x) {
  return x * x;
}
template <class T, class X>
inline T __pow(T a, X y) {
  T z = 1;
  for (int i = 1; i <= y; i++) {
    z *= a;
  }
  return z;
}
template <class T>
inline T gcd(T a, T b) {
  a = abs(a);
  b = abs(b);
  if (!b) return a;
  return __gcd(b, a % b);
}
template <class T>
inline T lcm(T a, T b) {
  a = abs(a);
  b = abs(b);
  return (a / __gcd(a, b)) * b;
}
template <class T>
void UpdateMin(T &x, T y) {
  if (y < x) {
    x = y;
  }
}
template <class T>
void UpdateMax(T &x, T y) {
  if (x < y) {
    x = y;
  }
}
template <class T, class X, class Y>
inline T bigmod(T n, X m, Y mod) {
  unsigned long long ret = 1, a = n % mod;
  while (m) {
    if (m & 1) ret = (ret * a) % mod;
    m >>= 1;
    a = (a * a) % mod;
  }
  ret %= mod;
  return (T)ret;
}
template <class T, class Y>
inline T modinv(T n, Y mod) {
  return bigmod(n, mod - 2, mod);
}
template <class T, class X>
int getbit(T s, X i) {
  return (s >> i) & 1;
}
template <class T, class X>
T onbit(T s, X i) {
  return s | (T(1) << i);
}
template <class T, class X>
T offbit(T s, X i) {
  return s & (~(T(1) << i));
}
template <class T>
inline void read(T &n) {
  char c;
  for (c = getchar(); !(c >= '0' && c <= '9'); c = getchar())
    ;
  n = c - '0';
  for (c = getchar(); c >= '0' && c <= '9'; c = getchar()) n = n * 10 + c - '0';
}
double ncr[55][55];
void precal() {
  for (int i = 0; i <= 50; i++) {
    for (int j = 0; j <= i; j++) {
      if (i == j || j == 0) {
        ncr[i][j] = 1;
      } else
        ncr[i][j] = ncr[i - 1][j - 1] + ncr[i - 1][j];
    }
  }
}
long long n, m;
double dp[55][55][55];
long long a[55];
double go(int p, int rem, int lm) {
  if (rem < 0) return 0;
  if (p == 0) return (rem == 0);
  double &ret = dp[p][rem][lm];
  if (!(ret == ret)) {
    long long tot = min(a[p] * lm, (long long)rem);
    ret = 0.0;
    for (int i = 0; i <= tot; i++) {
      ret += (ncr[rem][i] * go(p - 1, rem - i, lm));
    }
  }
  return ret;
}
double ans[55];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  ;
  precal();
  cin >> n >> m;
  for (int i = 1; i <= m; i++) cin >> a[i];
  memset(dp, -1, sizeof dp);
  for (int i = 1; i <= n; i++) {
    ans[i] = go(m, n, i);
  }
  double tot = go(m, n, n);
  double res = 0;
  for (int i = 1; i <= n; i++) {
    res += (ans[i] - ans[i - 1]) * (double)i / tot;
  }
  cout << setprecision(15) << fixed << res << endl;
  return 0;
}
