#include <bits/stdc++.h>
using namespace std;
int n, m, a[110];
double f[110][110][110], C[110][110];
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) scanf("%d", &a[i]);
  C[0][0] = 1;
  for (int i = 1; i <= 55; i++) {
    C[i][0] = 1;
    for (int j = 1; j <= i; j++) C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
  }
  for (int i = 0; i <= n; i++) f[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] == 0) ? (c / a[i]) : (c / a[i] + 1));
          f[i][j][k] +=
              f[i - 1][j - c][_max] * pow(i - 1, j - c) / pow(i, j) * C[j][c];
        }
  printf("%.10f\n", f[m][n][0]);
  return 0;
}
