#include <bits/stdc++.h>
using namespace std;
inline int add(int a, int b, int m = 1000000007) {
  a += b;
  if (a >= m) a -= m;
  return a;
}
inline int mul(int a, int b, int m = 1000000007) {
  return (int)(((long long)a * (long long)b) % m);
}
inline int norm(int x, int m = 1000000007) {
  if (x >= m) x %= m;
  if (x < 0) x += m;
  return x;
}
inline int neg(int x, int m = 1000000007) {
  x = -x;
  return norm(x);
}
long double p[51][51][51];
long double c[51][51];
int a[51];
long double f(int n, int m, int mx) {
  if (m == 0) return n == 0 ? mx : 0;
  long double &ans = p[n][m][mx];
  if (ans > -1) return ans;
  int here = 0;
  ans = 0;
  for (int i = 0; i <= n; ++i) {
    here = (int)(ceil((i + (long double).0) / a[m]) + 0.5);
    if (m != 1)
      ans += c[n][i] * pow((1.0 / m), (long double)i) *
             pow((m - (long double)1.0) / m, n - i) *
             f(n - i, m - 1, max(here, mx));
    else
      ans += c[n][i] * pow((1.0 / m), (long double)i) * (n - i == 0) *
             f(n - i, m - 1, max(here, mx));
  }
  return ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n, m;
  cin >> n >> m;
  for (int i = 1; i <= m; ++i) cin >> a[i];
  c[0][0] = 1;
  memset(p, -1, sizeof(p));
  for (int i = 1; i <= 50; ++i) {
    c[i][0] = 1;
    for (int j = 1; j <= i; ++j) c[i][j] = c[i - 1][j] + c[i - 1][j - 1];
  }
  cout << setprecision(20) << f(n, m, 0);
  return 0;
}
