#include <bits/stdc++.h>
using namespace std;
int n, m, a[60];
double ans = 0, b[110], c[110][110], f[110][110][110];
int main() {
  scanf("%d%d", &n, &m);
  b[0] = 1;
  for (int i = 1; i <= n; i++) b[i] = b[i - 1] / m;
  for (int i = 0; i <= n + m; i++) c[i][0] = c[i][i] = 1;
  for (int i = 1; i <= n + m; i++)
    for (int j = 1; j <= i; j++) c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
  f[0][0][0] = 1;
  for (int i = 1; i <= m; i++) {
    int num;
    f[i][0][0] = 1;
    scanf("%d", &num);
    for (int j = 1; j <= n; j++)
      for (int k = 1; k <= j; k++) {
        int l = max((k - 1) * num + 1, 0), r = k * num;
        for (int p = 0; p < l && p <= j; p++)
          f[i][j][k] += f[i - 1][j - p][k] * c[n - j + p][p];
        for (int p = l; p <= r && p <= j; p++)
          for (int t = 0; t <= k; t++)
            f[i][j][k] += f[i - 1][j - p][t] * c[n - j + p][p];
      }
  }
  for (int i = 1; i <= n; i++) ans += f[m][n][i] * i;
  printf("%.9lf", ans / pow(m, n));
  return 0;
}
