#include <bits/stdc++.h>
using namespace std;
int a[53];
int N, M;
double memo[53][53][53];
double comb[53][53];
void buildComb() {
  for (int i = 0; i <= 53; i++) {
    for (int j = 0; j <= 53; j++) {
      if (j == 0)
        comb[i][j] = 1;
      else if (i == 0)
        comb[i][j] = 0;
      else
        comb[i][j] = comb[i - 1][j - 1] + comb[i - 1][j];
    }
  }
}
double calcMemo(int r, int s, int mxQ) {
  if (r == M) {
    if (s) return 0;
    return mxQ;
  }
  double &ret = memo[r][s][mxQ];
  if (ret == ret) return ret;
  ret = 0;
  for (int i = 0; i <= s; i++) {
    int yy = ((i + a[r] - 1) / a[r]);
    ret += comb[s][i] * calcMemo(r + 1, s - i, max(yy, mxQ));
  }
  return ret;
}
int main() {
  ios::sync_with_stdio(0);
  cout.tie(NULL);
  cin.tie(NULL);
  buildComb();
  cin >> N >> M;
  for (int i = 0; i < M; i++) cin >> a[i];
  memset(memo, -1, sizeof memo);
  cout.precision(9);
  cout << fixed << calcMemo(0, N, 0) / pow(M, N) << "\n";
  return 0;
}
