#include <bits/stdc++.h>
using namespace std;
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cerr << name << " : " << arg1 << std::endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
long long n, m, a[51];
long double dp[51][51][51];
long double C[51][51];
long double f(long long pos, long long rem, long long mx) {
  if (pos >= m) return rem ? 0 : mx;
  if (rem < 0) return 0;
  if (dp[pos][rem][mx] > -0.5) return dp[pos][rem][mx];
  long double ans = 0;
  for (long long i = 0; i < rem + 1; ++i)
    ans += f(pos + 1, rem - i, max(mx, i / a[pos] + (bool)(i % a[pos]))) *
           C[rem][i];
  return dp[pos][rem][mx] = ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  for (long long i = 0; i < 51; ++i)
    for (long long j = 0; j < 51; ++j)
      for (long long k = 0; k < 51; ++k) dp[i][j][k] = -1.0;
  cin >> n >> m;
  for (long long i = 0; i < m; ++i) cin >> a[i];
  for (long long i = 0; i < 51; ++i) C[i][0] = 1;
  for (long long i = 1; i < 51; ++i)
    for (long long j = 1; j < 51; ++j)
      C[i][j] = C[i - 1][j] + C[i - 1][j - 1] / (long double)m;
  cout << setprecision(20) << f(0, n, 0);
  return 0;
}
