#include <iostream>
#include <iomanip>
#include <cstdio>
#include <queue>
#include <cstring>
#include <vector>
#include <map>
#include <algorithm>
#include <cctype>
#include <cmath>
#include <bitset>
#include <set>
#include <deque>
#include <stack>
#include <atcoder/all>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < (t); ++i)
#define dsrep(i,t,s) for(int i = (t)-1; i >= (s); --i)
#define rng(a) a.begin(),a.end()
#define rrng(a) a.rbegin(),a.rend()
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
int dx4[4]={1,0,-1,0};
int dy4[4]={0,-1,0,1};
using ll = long long;
using uint = unsigned;
using ull = unsigned long long;
using P = pair<int,int>;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
// const ll mod = 998244353;
const ll mod = 1e9 + 7;

////////////////////////////////////////////
int n, m;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  cin >> n >> m;

  vector<int> a(m);
  rep(i, m) cin >> a[i];

  double plus = 0.0;
  plus += a[0] - 1; plus += n - a[m - 1];

  double l = 0, r = 1000000000, ans;
  int cnt = 50;
  rep(i, cnt) {
    double mid = (l + r) / 2;
    vector<double> dp(m , 0);
    double sum = 0;

    for (int i = m - 2; i >= 0; i--) {
      dp[i] = min(dp[i + 1] + a[i + 1] - a[i], mid + 1);
      sum += dp[i];
    }

    if(sum / m >= mid) l = mid;
    else r = mid;

    if(i == cnt - 1) ans = dp[0];
  }

  cout << fixed << setprecision(10) << (ans + plus) << endl;
}