#include <bits/stdc++.h>
using namespace std;
struct IO {
  char buf[(1 << 20)], *p1, *p2;
  char pbuf[(1 << 20)], *pp;
  IO() : p1(buf), p2(buf), pp(pbuf) {}
  inline char gc() {
    return getchar();
    if (p1 == p2) p2 = (p1 = buf) + fread(buf, 1, (1 << 20), stdin);
    return p1 == p2 ? ' ' : *p1++;
  }
  inline bool blank(char ch) {
    return ch == ' ' || ch == '\n' || ch == '\r' || ch == '\t';
  }
  template <class T>
  inline void read(T &x) {
    register double tmp = 1;
    register bool sign = 0;
    x = 0;
    register char ch = gc();
    for (; !(ch >= '0' && ch <= '9'); ch = gc())
      if (ch == '-') sign = 1;
    for (; (ch >= '0' && ch <= '9'); ch = gc()) x = x * 10 + (ch - '0');
    if (ch == '.')
      for (ch = gc(); (ch >= '0' && ch <= '9'); ch = gc())
        tmp /= 10.0, x += tmp * (ch - '0');
    if (sign) x = -x;
  }
  inline void read(char *s) {
    register char ch = gc();
    for (; blank(ch); ch = gc())
      ;
    for (; !blank(ch); ch = gc()) *s++ = ch;
    *s = 0;
  }
  inline void read(char &c) {
    for (c = gc(); blank(c); c = gc())
      ;
  }
  template <class t>
  inline void write(t x) {
    if (x < 0)
      putchar('-'), write(-x);
    else {
      if (x > 9) write(x / 10);
      putchar('0' + x % 10);
    }
  }
} io;
const int mod = 1e9 + 7;
const int mo = 998244353;
const int N = 55;
int n, m;
double C[N][N], f[N][N][N];
int a[N];
signed main() {
  io.read(n), io.read(m);
  for (int i = (1); i <= (m); i++) io.read(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 <= (n); j++) C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
  f[0][0][0] = 1;
  for (int i = (1); i <= (m); i++)
    for (int j = (0); j <= (n); j++)
      for (int k = (0); k <= (j); k++)
        for (int l = (0); l <= (n - j); l++)
          f[i][j + l][max(((l + a[i] - 1) / a[i]), k)] +=
              f[i - 1][j][k] * C[n - j][l];
  double ans = 0, gs = 0;
  for (int i = (0); i <= (n); i++) gs += f[m][n][i], ans += f[m][n][i] * i;
  printf("%.12lf\n", 1.0 * ans / (1.0 * gs));
  return 0;
}
