#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
const int mod = 1e9 + 7;
int n, m;
int a[56];
double dp[56][56][56];
double C[56][56];
int main() {
  cin >> n >> m;
  C[0][0] = 1.0;
  for (int i = 1; i <= 55; i++) {
    C[i][0] = 1.0;
    for (int j = 1; j <= i; j++) {
      C[i][j] = C[i - 1][j - 1] + C[i - 1][j];
    }
  }
  for (int i = 1; i <= m; i++) cin >> a[i];
  for (int i = 0; i <= n; i++) dp[0][0][i] = i;
  for (int i = 1; i <= m; i++) {
    for (int j = 0; j <= n; j++) {
      for (int k = 0; k <= n; k++) {
        for (int c = 0; c <= j; c++) {
          int Max = max(k, (c + a[i] - 1) / a[i]);
          dp[i][j][k] +=
              dp[i - 1][j - c][Max] * pow(i - 1, j - c) / pow(i, j) * C[j][c];
        }
      }
    }
  }
  printf("%.10f\n", dp[m][n][0]);
  return 0;
}
