#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
const int mod = 1e9 + 7;
int n, m, a[56];
double pro[56];
double dp[56][56], C[56][56];
int main() {
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    cin >> a[i];
  }
  for (int i = 0; i <= n; i++) {
    C[i][0] = 1;
    for (int j = 1; j <= i; j++) {
      C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
    }
  }
  pro[0] = 0;
  for (int max = 1; max <= n; max++) {
    memset(dp, 0, sizeof(dp));
    dp[0][0] = 1;
    for (int i = 1; i <= m; i++) {
      for (int j = 0; j <= n; j++) {
        for (int k = 0; k <= j and k <= a[i] * max; k++) {
          dp[i][j] += dp[i - 1][j - k] * C[j][k];
        }
      }
    }
    pro[max] = dp[m][n];
    for (int i = 1; i <= n; i++) {
      pro[max] /= m;
    }
  }
  double res = 0;
  for (int i = 1; i <= n; i++) {
    res += (pro[i] - pro[i - 1]) * i;
  }
  printf("%.10f\n", res);
  return 0;
}
