#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <queue>
#include <utility>
#include <tuple>
#include <cmath>
#include <numeric>
#include <set>
#include <map>
#include <array>
#include <complex>
#include <iomanip>
#include <cassert>
#include <random>
using ll = long long;
using std::cin;
using std::cout;
using std::endl;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
const int inf = (int)1e9 + 7;
const long long INF = 1LL << 60;

void solve()
{
  int n, m; cin >> n >> m;
  std::vector<int> b(m);
  for (int i = 0; i < m; ++i)
  {
    cin >> b[i];
  }
  using real = long double;
  std::vector<real> dp(m);
  dp.back() = n - b.back();
  real sum = dp.back();
  for (int i = m - 2; i >= 0; --i)
  {
    real pr = (real)(m - i - 1) / (real)m;
    dp[i] = std::min<real>((1 + sum / (real)m) / pr, n - b[i]);
    sum += dp[i];
  }
  real res = 1e18;
  for (int i = 0; i < m; ++i)
  {
    chmin(res, dp[i] + b[i] - 1);
  }
  cout << std::fixed << std::setprecision(15) << res << "\n";
}
int main()
{
  std::cin.tie(nullptr);
  std::ios::sync_with_stdio(false);

  int kkt = 1;
  //cin >> kkt;
  while(kkt--)
    solve();
  return 0;
}