#ifdef __LOCAL
#define _GLIBCXX_DEBUG
#endif
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int,int>;
using PIL = pair<int,ll>;
using PLI = pair<ll,int>;
using PLL = pair<ll,ll>;
using Graph = vector<vector<int>>;
using Cost_Graph = vector<vector<PIL>>;

template<class T> bool chmin(T &a, T b) {if(a>b){a=b;return 1;}return 0;}
template<class T> bool chmax(T &a, T b) {if(a<b){a=b;return 1;}return 0;}
template<class T> void show_vec(T v) {for (int i=0;i<v.size();i++) cout<<v[i]<<endl;}
template<class T> void show_pair(T p) {cout<<p.first<<" "<<p.second<<endl;}
template<class T> bool judge_digit(T bit,T i) {return (((bit&(1LL<<i))!=0)?1:0);}
#define REP(i,n) for(int i=0;i<int(n);i++)
#define ROUNDUP(a,b) (((a)+(b)-1)/(b))
#define YESNO(T) cout<<(T?"YES":"NO")<<endl
#define yesno(T) cout<<(T?"yes":"no")<<endl
#define YesNo(T) cout<<(T?"Yes":"No")<<endl

const int INFint = 1 << 29;
const ll INF = 1LL << 40;
const ll MOD = 1000000007LL;
const double pi = 3.14159265358979;
const vector<int> h_idx4 = {-1, 0,0,1};
const vector<int> w_idx4 = { 0,-1,1,0};
const vector<int> h_idx8 = {-1,-1,-1, 0,0, 1,1,1};
const vector<int> w_idx8 = {-1, 0, 1,-1,1,-1,0,1};

double seg_op(double a,double b){ return min(a,b); }
double seg_e(){ return (double)INF; }
template <class S, S (*op)(S, S), S (*e)()> struct segtree {
  public:
    segtree() : segtree(0) {}
    segtree(int n) : segtree(std::vector<S>(n, e())) {}
    segtree(const std::vector<S>& v) : _n(int(v.size())) {
        log = ceil_pow2(_n);
        size = 1 << log;
        d = std::vector<S>(2 * size, e());
        for (int i = 0; i < _n; i++) d[size + i] = v[i];
        for (int i = size - 1; i >= 1; i--) {
            update(i);
        }
    }
 
    void set(int p, S x) {
        assert(0 <= p && p < _n);
        p += size;
        d[p] = x;
        for (int i = 1; i <= log; i++) update(p >> i);
    }
 
    S get(int p) {
        assert(0 <= p && p < _n);
        return d[p + size];
    }
 
    S prod(int l, int r) {
        assert(0 <= l && l <= r && r <= _n);
        S sml = e(), smr = e();
        l += size;
        r += size;
 
        while (l < r) {
            if (l & 1) sml = op(sml, d[l++]);
            if (r & 1) smr = op(d[--r], smr);
            l >>= 1;
            r >>= 1;
        }
        return op(sml, smr);
    }
 
    S all_prod() { return d[1]; }
 
    template <bool (*f)(S)> int max_right(int l) {
        return max_right(l, [](S x) { return f(x); });
    }
    template <class F> int max_right(int l, F f) {
        assert(0 <= l && l <= _n);
        assert(f(e()));
        if (l == _n) return _n;
        l += size;
        S sm = e();
        do {
            while (l % 2 == 0) l >>= 1;
            if (!f(op(sm, d[l]))) {
                while (l < size) {
                    l = (2 * l);
                    if (f(op(sm, d[l]))) {
                        sm = op(sm, d[l]);
                        l++;
                    }
                }
                return l - size;
            }
            sm = op(sm, d[l]);
            l++;
        } while ((l & -l) != l);
        return _n;
    }
 
    template <bool (*f)(S)> int min_left(int r) {
        return min_left(r, [](S x) { return f(x); });
    }
    template <class F> int min_left(int r, F f) {
        assert(0 <= r && r <= _n);
        assert(f(e()));
        if (r == 0) return 0;
        r += size;
        S sm = e();
        do {
            r--;
            while (r > 1 && (r % 2)) r >>= 1;
            if (!f(op(d[r], sm))) {
                while (r < size) {
                    r = (2 * r + 1);
                    if (f(op(d[r], sm))) {
                        sm = op(d[r], sm);
                        r--;
                    }
                }
                return r + 1 - size;
            }
            sm = op(d[r], sm);
        } while ((r & -r) != r);
        return 0;
    }
 
  private:
    int _n, size, log;
    std::vector<S> d;
 
    void update(int k) { d[k] = op(d[2 * k], d[2 * k + 1]); }

    // @param n `0 <= n`
    // @return minimum non-negative `x` s.t. `n <= 2**x`
    int ceil_pow2(int n) {
        int x = 0;
        while ((1U << x) < (unsigned int)(n)) x++;
        return x;
    }
};

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(15);

    ll n,m;
    cin >> n >> m;
    vector<ll> b(m);
    for (int i = 0; i < m; i++){
        cin >> b[i];
    }

    vector<double> c(m);
    for (int i = 0; i < m; i++){
        c[i] = (double)(b[m - 1] - b[i]);
    }

    vector<double> s(m + 1);
    s[m] = 0;
    for (int i = m; i > 0; i--){
        s[i - 1] = s[i] + c[i - 1];
    }
    // show_vec(s);

    double dm = (double)m;

    double ans = c[0];
    for (int i = 1; i < m; i++){
        double di = (double)i;
        double t = (dm / (dm - di)) + ((1.0 / (dm - di)) * s[i]);

        chmin(ans,t);
    }

    cout << ans + (b[0] - 1) + (n - b[m - 1]) << endl;
}