#include <bits/stdc++.h>
using namespace std;
double ans, pre, f[55][55], p[55], s;
int n, m, a[55];
long long c[55][55];
void init() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) scanf("%d", &a[i]);
}
void work() {
  int i, j, k, kk;
  p[0] = 1;
  for (i = 1; i <= n; i++) p[i] = p[i - 1] * 1.0 / m;
  c[0][0] = 1;
  for (i = 1; i <= n; i++) {
    c[i][0] = 1;
    for (j = 1; j <= n; j++) c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
  }
  for (kk = 1; kk <= n; kk++) {
    for (i = 0; i <= m; i++)
      for (j = 0; j <= n; j++) f[i][j] = 0;
    f[0][0] = 1;
    for (i = 0; i < m; i++)
      for (j = 0; j <= n; j++)
        for (k = 0; k <= a[i + 1] * kk && j + k <= n; k++)
          f[i + 1][j + k] += f[i][j] * p[k] * c[n - j][k];
    ans += (f[m][n] - pre) * kk;
    pre = f[m][n];
  }
  printf("%.20lf\n", ans);
}
int main() {
  init();
  work();
  return 0;
}
