#include <bits/stdc++.h>
using namespace std;
using int128  = __int128_t;
using int64   = long long;
using int32   = int;
using uint128 = __uint128_t;
using uint64  = unsigned long long;
using uint32  = unsigned int;

#define ALL(obj) (obj).begin(),(obj).end()
template<class T> using priority_queue_reverse = priority_queue<T,vector<T>,greater<T>>;

constexpr int64 MOD = 1'000'000'000LL + 7; //'
constexpr int64 MOD2 = 998244353;
constexpr int64 HIGHINF = 1'000'000'000'000'000'000LL;
constexpr int64 LOWINF = 1'000'000'000'000'000LL; //'
constexpr long double PI = 3.1415926535897932384626433L;

template <class T> vector<T> multivector(size_t N,T init){return vector<T>(N,init);}
template <class... T> auto multivector(size_t N,T... t){return vector<decltype(multivector(t...))>(N,multivector(t...));}
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const multiset<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const deque<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max(a,b);}
template <class T> void chmin(T& a, const T b){a=min(a,b);}
vector<string> split(const string &str, const char delemiter) {vector<string> res;stringstream ss(str);string buffer; while( getline(ss, buffer, delemiter) ) res.push_back(buffer); return res;}
inline constexpr int msb(int x) {return x?31-__builtin_clz(x):-1;}
inline constexpr int64 ceil_div(const int64 a,const int64 b) {return (a+(b-1))/b;}// return ceil(a/b)
void YN(bool flg) {cout << (flg ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << (flg ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << (flg ? "yes" : "no") << endl;}

/**
 * @url 
 * @est
 */ 
int main() {
    cin.tie(0);ios::sync_with_stdio(false);
    int64 N,M; cin >> N >> M;
    vector<int64> B(M);
    for(int i=0;i<M;++i) cin >> B[i];
    corner(N==1,0);
    vector<int64> A,C;
    int64 p=1;
    int flg = 0;
    if(B.back()!=N) {
        B.push_back(N);
        flg = 1;
    }
    for(int i=0;i<B.size();++i) {
        if(p==B[i]) {
            C.push_back(1);
            A.push_back(1);
            ++p;
        }
        else {
            C.push_back(0);
            A.push_back(B[i]-p);
            p = B[i];
            C.push_back(1);
            A.push_back(1);
            ++p;
        }
    }
    if(flg) B.pop_back();
    int L = A.size();

    double l = 0, r = LOWINF, m;
    double ans;
    for(int n=0;n<100;++n) {
        m = (l+r) / 2;
        vector<double> dp(L);
        dp[L-1] = 0;
        for(int i=L-2; 0<=i; --i) {
            if(C[i]) {
                dp[i] = A[i] + min(dp[i+1],m);
            } 
            else {
                dp[i] = A[i] + dp[i+1];
            }
        }
        double sum = 0;
        for(int i=0;i<L;++i) if(C[i]) sum += dp[i];
        sum /= M;
        if(sum <= m) {
            r = m;
            ans = dp[0];
        }
        else {
            l = m;
        }
    }
    printf("%.10f\n",ans);
    return 0;
}