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