#include <bits/stdc++.h>

using namespace std;

#define int long long
#define rep(i, n) for(int (i)=0;(i)<(n);(i)++)
#define rrep(i, n) for(int (i)=((n)-1);(i)>=0;(i)--)
#define itn int
#define miele(v) min_element(v.begin(), v.end())
#define maele(v) max_element(v.begin(), v.end())
#define SUM(v) accumulate(v.begin(), v.end(), 0.0)
#define lb(a, key) lower_bound(a.begin(),a.end(),key)
#define ub(a, key) upper_bound(a.begin(),a.end(),key)
#define COUNT(a, key) count(a.begin(), a.end(), key)
#define BITCOUNT(x) __builtin_popcount(x)
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define F first
#define S second
using P = pair<int, int>;
using WeightedGraph = vector<vector<P>>;
using UnWeightedGraph = vector<vector<int>>;
using Real = long double;
using Point = complex<Real>; //Point and Vector2d is the same!
// p.real() or real(p) -> x軸, p.imag() or imag(p) -> y軸
using Vector2d = complex<Real>;
const int MOD = 1000000007;
const long long INF = 1LL << 60;
const double EPS = 1e-15;
const double PI = 3.14159265358979323846;

template<typename T>
int getIndexOfLowerBound(vector<T> &v, T x) {
    return lower_bound(v.begin(), v.end(), x) - v.begin();
}

template<typename T>
int getIndexOfUpperBound(vector<T> &v, T x) {
    return upper_bound(v.begin(), v.end(), x) - v.begin();
}

template<class T>
inline bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
inline bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)

istream &operator>>(istream &is, Point &p) {
    Real a, b;
    is >> a >> b;
    p = Point(a, b);
    return is;
}

template<typename T, typename U>
istream &operator>>(istream &is, pair<T, U> &p_var) {
    is >> p_var.first >> p_var.second;
    return is;
}

template<typename T>
istream &operator>>(istream &is, vector<T> &vec) {
    for (T &x : vec) is >> x;
    return is;
}

template<typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
    os << pair_var.first << ' ' << pair_var.second;
    return os;
}

template<typename T>
ostream &operator<<(ostream &os, vector<T> &vec) {
    for (int i = 0; i < vec.size(); i++)
        os << vec[i] << ' ';
    return os;
}

template<typename T, typename U>
ostream &operator<<(ostream &os, vector<pair<T, U>> &vec) {
    for (int i = 0; i < vec.size(); i++)
        os << vec[i] << '\n';
    return os;
}

template<typename T>
ostream &operator<<(ostream &os, vector<vector<T>> &df) {
    for (auto &vec : df) os << vec;
    return os;
}

template<typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
    repi(itr, map_var) {
        os << *itr << ' ';
        itr++;
        itr--;
    }
    return os;
}

template<typename T>
ostream &operator<<(ostream &os, set<T> &set_var) {
    repi(itr, set_var) {
        os << *itr << ' ';
        itr++;
        itr--;
    }
    return os;
}

void print() { cout << endl; }

template<class Head, class... Tail>
void print(Head &&head, Tail &&... tail) {
    cout << head;
    if (sizeof...(tail) != 0) cout << " ";
    print(forward<Tail>(tail)...);
}

long double ans = 0;

bool isOk(long double x, vector <long double> &b) {
    int n = b.size();
    vector <long double> dp(b.size());
    long double sum = 0;
    rrep(i, n-1) {
        sum += dp[i] = min(dp[i+1] + b[i+1] - b[i], 1.0 + x);
    }

    ans = dp[0];

    if(sum/n > x) return false;
    else return true;
}

signed main(void) { cin.tie(0); ios::sync_with_stdio(false);
    int n, m; cin>>n>>m;
    vector <long double> b(m);

    cin>>b;

    long double ng = -1, ok = 1145141919; //範囲にめっちゃ注意!
    while (abs(ok-ng) > 0.000001) {
        long double mid = ng + (ok - ng) / 2.0;
        if(isOk(mid, b)) {
            ok = mid;
        } else {
            ng = mid;
        }
    }

    printf("%.10Lf\n",  ans + (b[0] - 1) + (n - b[m-1]));
}