#include <bits/stdc++.h>
using namespace std;
const int N = 55 + 7;
const int M = 1e6 + 1;
const int base = 1e9 + 7;
const double ep = 1e-9;
long double dp[N][N][N];
long double sum[N][N][N];
long double C[N][N];
int a[N];
int n, m;
int main() {
  cin >> n >> m;
  for (int i = 0; i < m; i++) cin >> a[i + 1];
  C[0][0] = 1;
  for (int i = 1; i < n + 1; i++) {
    C[i][0] = C[i][i] = 1;
    for (int j = 1; j < i; j++) C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
  }
  dp[0][0][0] = 1;
  for (int i = 0; i < n + 1; i++) sum[0][i][0] = 1;
  for (int i = 1; i < m + 1; i++) {
    dp[i][0][0] = 1;
    sum[i][0][0] = 1;
    for (int t = 1; t < n + 1; t++) {
      for (int u = (t - 1) * a[i] + 1; u < min(t * a[i] + 1, n + 1); u++)
        for (int v = u; v < n + 1; v++) {
          dp[i][t][v] += sum[i - 1][t][v - u] * C[v][v - u];
        }
      for (int v = 0; v < n + 1; v++)
        for (int u = 0; u < v + 1; u++)
          if (v - u <= (t - 1) * a[i])
            dp[i][t][v] += dp[i - 1][t][u] * C[v][v - u];
      for (int v = 0; v < n + 1; v++)
        sum[i][t][v] = sum[i][t - 1][v] + dp[i][t][v];
    }
  }
  long double res = 0;
  for (int t = 0; t < n + 1; t++) {
    res += dp[m][t][n];
  }
  double ans = 0;
  for (int t = 0; t < n + 1; t++) ans += (double)dp[m][t][n] / res * t;
  printf("%0.9lf", ans);
}
