#include <bits/stdc++.h>
using namespace std;
const int INF = 1000000000;
const int eps = 1e-11;
const int N = 52;
int a[N], n, m;
double dp[N][N][N], c[N][N];
int main() {
  int t;
  c[0][0] = 1.0;
  for (int i = 1; i < N; i++) {
    c[i][0] = 1.0;
    for (int j = 1; j <= i; j++) c[i][j] = c[i - 1][j] + c[i - 1][j - 1];
  }
  while (scanf("%d%d", &n, &m) != EOF) {
    for (int i = 1; i <= m; i++) scanf("%d", &a[i]);
    memset(dp, 0, sizeof(dp));
    dp[0][0][0] = 1.0;
    for (int i = 0; i < m; i++)
      for (int j = 0; j <= n; j++) {
        for (int k = 0; k <= n; k++)
          if (dp[i][j][k] > eps) {
            for (int h = 0; j + h <= n; h++) {
              int t = h / a[i + 1] + (h % a[i + 1] != 0);
              t = max(t, k);
              dp[i + 1][j + h][t] +=
                  pow(2.0, log2(dp[i][j][k]) + log2(c[n - j][h]) - h * log2(m));
            }
          }
      }
    double ans = 0;
    for (int j = 1; j <= n; j++) ans += dp[m][n][j] * j;
    printf("%.18f\n", ans);
  }
  return 0;
}
