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