#include 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; using WeightedGraph = vector>; using UnWeightedGraph = vector>; using Real = long double; using Point = complex; //Point and Vector2d is the same! // p.real() or real(p) -> x軸, p.imag() or imag(p) -> y軸 using Vector2d = complex; const int MOD = 1000000007; const long long INF = 1LL << 60; const double EPS = 1e-15; const double PI = 3.14159265358979323846; template int getIndexOfLowerBound(vector &v, T x) { return lower_bound(v.begin(), v.end(), x) - v.begin(); } template int getIndexOfUpperBound(vector &v, T x) { return upper_bound(v.begin(), v.end(), x) - v.begin(); } template inline bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; } template 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 istream &operator>>(istream &is, pair &p_var) { is >> p_var.first >> p_var.second; return is; } template istream &operator>>(istream &is, vector &vec) { for (T &x : vec) is >> x; return is; } template ostream &operator<<(ostream &os, pair &pair_var) { os << pair_var.first << ' ' << pair_var.second; return os; } template ostream &operator<<(ostream &os, vector &vec) { for (int i = 0; i < vec.size(); i++) os << vec[i] << ' '; return os; } template ostream &operator<<(ostream &os, vector> &vec) { for (int i = 0; i < vec.size(); i++) os << vec[i] << '\n'; return os; } template ostream &operator<<(ostream &os, vector> &df) { for (auto &vec : df) os << vec; return os; } template ostream &operator<<(ostream &os, map &map_var) { repi(itr, map_var) { os << *itr << ' '; itr++; itr--; } return os; } template ostream &operator<<(ostream &os, set &set_var) { repi(itr, set_var) { os << *itr << ' '; itr++; itr--; } return os; } void print() { cout << endl; } template void print(Head &&head, Tail &&... tail) { cout << head; if (sizeof...(tail) != 0) cout << " "; print(forward(tail)...); } long double ans = 0; bool isOk(long double x, vector &b) { int n = b.size(); vector 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 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])); }