#include <bits/stdc++.h>
using namespace std;
const int N = 55;
bool visit[N][N][N];
double mem[N][N][N];
int n, m, arr[N];
long long int c[N][N];
void pre() {
  c[0][0] = 1;
  for (int i = 1; i < N; i++) {
    c[i][0] = 1;
    for (int j = 1; j < i; j++) {
      c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
    }
    c[i][i] = 1;
  }
}
double f(int j, int rem, int mx) {
  if (j == m && rem == 0) return mx;
  if (j == m) return 0;
  if (visit[j][rem][mx]) return mem[j][rem][mx];
  visit[j][rem][mx] = 1;
  double &ret = mem[j][rem][mx], p = 1;
  for (int i = 0; i <= rem; i++) {
    ret +=
        f(j + 1, rem - i, max(mx, (i + (arr[j] - 1)) / arr[j])) * p * c[rem][i];
    p /= m;
  }
  return ret;
}
int main() {
  pre();
  cin >> n >> m;
  for (int i = 0; i < m; i++) cin >> arr[i];
  printf("%.10f\n", f(0, n, 0));
  return 0;
}
