#include <bits/stdc++.h>
using namespace std;
const int maxn = 55;
double dp[maxn][maxn][maxn], C[maxn][maxn];
int a[maxn];
double qp(double x, int p) {
  double res = 1.0;
  while (p) {
    if (p & 1) res = res * x;
    x = x * x;
    p >>= 1;
  }
  return res;
}
int main() {
  for (int i = 0; i <= 50; i++) C[i][0] = C[i][i] = 1.0;
  for (int i = 1; i <= 50; i++) {
    for (int j = 1; j < i; j++) C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
  }
  int n, m;
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) scanf("%d", &a[i]);
  dp[0][n][0] = 1.0;
  for (int i = 0; i < m; i++) {
    for (int j = 0; j <= n; j++) {
      for (int k = 0; k <= (n - j); k++) {
        for (int c = 0; c <= j; c++) {
          dp[i + 1][j - c][max(k, (c + a[i + 1] - 1) / a[i + 1])] +=
              dp[i][j][k] * C[j][c] / qp(1.0 * m, c);
        }
      }
    }
  }
  double ans = 0;
  for (int i = 1; i <= n; i++) ans += 1.0 * i * dp[m][0][i];
  printf("%.12f\n", ans);
  return 0;
}
