#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <climits>
#include <map>
#include <queue>
#include <set>
#include <cstring>
#include <vector>

using namespace std;
typedef long long ll;

int N, M;
vector<int> B;

double f(int mid) {
  if (mid < 0) return INT_MAX;
  if (M <= mid) return INT_MAX;

  int len = M - mid;
  double val = 0.0;

  for (int i = M - 1; i >= mid; --i) {
    int b = B[i];
    double cnt = (B[0] - 1) + M * 1.0 / len + (N - b);
    val += cnt;
  }

  return val / len;
}

int main() {
  cin >> N >> M;
  for (int i = 0; i < M; ++i) {
    int b;
    cin >> b;
    B.push_back(b);
  }

  int l = 0;
  int r = M - 1;

  for (int i = 0; i < 10000; ++i) {
    int i_1 = (l * 2 + r) / 3;
    int i_2 = (l + r * 2) / 3;

    if (f(i_1) < f(i_2)) {
      r = i_1;
    } else {
      l = i_2;
    }
  }

  int mid = (l + r) / 2;
  double ans = N - 1.0;
  ans = min(ans, f(mid - 1));
  ans = min(ans, f(mid));
  ans = min(ans, f(mid + 1));
  cout << ans << endl;

  return 0;
}