#include <bits/stdc++.h>
const int MAXN = 3e5 + 10;
const double eps = 1e-8;
using namespace std;
struct edge {
  int t, v;
  edge *next;
} e[MAXN << 1], *h[MAXN], *o = e;
void add(int x, int y, int vul) {
  o->t = y;
  o->v = vul;
  o->next = h[x];
  h[x] = o++;
}
long long read() {
  long long x = 0, f = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) x = x * 10 + ch - '0', ch = getchar();
  return x * f;
}
int n, m, a[MAXN];
double dp[51][51][51], num[51][51];
double calc(int x, int y) {
  if (x < y) return 0;
  return num[x][y];
}
int main() {
  n = read();
  m = read();
  for (int i = 0; i <= n; i++) num[i][0] = 1;
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= i; j++) num[i][j] = num[i - 1][j - 1] + num[i - 1][j];
  for (int i = 1; i <= m; i++) a[i] = read();
  for (int i = 0; i <= m; i++) dp[i][0][0] = 1;
  for (int i = 1; i <= m; i++) {
    for (int j = 1; j <= n; j++) {
      for (int k = 1; k <= n; k++) {
        for (int p = (k - 1) * a[i] + 1; p <= k * a[i]; p++) {
          if (p <= j) {
            for (int l = 0; l <= k; l++)
              dp[i][j][k] += dp[i - 1][j - p][l] * calc(n - j + p, p);
          }
        }
        for (int p = 0; p <= (k - 1) * a[i]; p++) {
          if (j >= p) dp[i][j][k] += dp[i - 1][j - p][k] * calc(n - j + p, p);
        }
      }
    }
  }
  double ans = 0;
  for (int i = 1; i <= n; i++) ans += dp[m][n][i] * 1.0 * i;
  for (int i = 1; i <= n; i++) ans /= m;
  printf("%.10f\n", ans);
  return 0;
}
