#include using namespace std; typedef long long ll; #define pb(x) push_back(x) #define mp(a, b) make_pair(a, b) #define all(x) x.begin(), x.end() #define rall(x) x.rbegin(), x.rend() #define lscan(x) scanf("%I64d", &x) #define lprint(x) printf("%I64d", x) #define rep(i, n) for (ll i = 0; i < (n); i++) #define rep2(i, n) for (ll i = n - 1; i >= 0; i--) #define REP(i, l, r) for (ll i = l; i < (r); i++) template using rque = priority_queue, greater>; const ll mod = 998244353; template bool chmin(T &a, const T &b) { if (b < a) { a = b; return 1; } return 0; } template bool chmax(T &a, const T &b) { if (b > a) { a = b; return 1; } return 0; } ll gcd(ll a, ll b) { ll c = a % b; while (c != 0) { a = b; b = c; c = a % b; } return b; } long long extGCD(long long a, long long b, long long &x, long long &y) { if (b == 0) { x = 1; y = 0; return a; } long long d = extGCD(b, a % b, y, x); y -= a / b * x; return d; } struct UnionFind { vector data; UnionFind(int sz) { data.assign(sz, -1); } bool unite(int x, int y) { x = find(x), y = find(y); if (x == y) return (false); if (data[x] > data[y]) swap(x, y); data[x] += data[y]; data[y] = x; return (true); } int find(int k) { if (data[k] < 0) return (k); return (data[k] = find(data[k])); } ll size(int k) { return (-data[find(k)]); } }; ll M = 1000000007; vector fac(2000011, 0); //n!(mod M) vector ifac(2000011); //k!^{M-2} (mod M) ll mpow(ll x, ll n) { ll ans = 1; while (n != 0) { if (n & 1) ans = ans * x % M; x = x * x % M; n = n >> 1; } return ans; } ll mpow2(ll x, ll n, ll mod) { ll ans = 1; while (n != 0) { if (n & 1) ans = ans * x % mod; x = x * x % mod; n = n >> 1; } return ans; } void setcomb() { fac[0] = 1; ifac[0] = 1; for (ll i = 0; i < 2000010; i++) { fac[i + 1] = fac[i] * (i + 1) % M; // n!(mod M) } ifac[2000010] = mpow(fac[2000010], M - 2); for (ll i = 2000010; i > 0; i--) { ifac[i - 1] = ifac[i] * i % M; } } ll comb(ll a, ll b) { if(fac[0] == 0) setcomb(); if (a == 0 && b == 0) return 1; if (a < b || a < 0) return 0; ll tmp = ifac[a - b] * ifac[b] % M; return tmp * fac[a] % M; } ll perm(ll a, ll b) { if (a == 0 && b == 0) return 1; if (a < b || a < 0) return 0; return fac[a] * ifac[a - b] % M; } long long modinv(long long a) { long long b = M, u = 1, v = 0; while (b) { long long t = a / b; a -= t * b; swap(a, b); u -= t * v; swap(u, v); } u %= M; if (u < 0) u += M; return u; } ll modinv2(ll a, ll mod) { ll b = mod, u = 1, v = 0; while (b) { ll t = a / b; a -= t * b; swap(a, b); u -= t * v; swap(u, v); } u %= mod; if (u < 0) u += mod; return u; } template struct ModInt { int x; ModInt() : x(0) {} ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {} ModInt &operator+=(const ModInt &p) { if ((x += p.x) >= mod) x -= mod; return *this; } ModInt &operator-=(const ModInt &p) { if ((x += mod - p.x) >= mod) x -= mod; return *this; } ModInt &operator*=(const ModInt &p) { x = (int)(1LL * x * p.x % mod); return *this; } ModInt &operator/=(const ModInt &p) { *this *= p.inverse(); return *this; } ModInt operator-() const { return ModInt(-x); } ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; } ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; } ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; } ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; } bool operator==(const ModInt &p) const { return x == p.x; } bool operator!=(const ModInt &p) const { return x != p.x; } ModInt inverse() const { int a = x, b = mod, u = 1, v = 0, t; while (b > 0) { t = a / b; swap(a -= t * b, b); swap(u -= t * v, v); } return ModInt(u); } ModInt pow(int64_t n) const { ModInt ret(1), mul(x); while (n > 0) { if (n & 1) ret *= mul; mul *= mul; n >>= 1; } return ret; } friend ostream &operator<<(ostream &os, const ModInt &p) { return os << p.x; } friend istream &operator>>(istream &is, ModInt &a) { int64_t t; is >> t; a = ModInt(t); return (is); } static int get_mod() { return mod; } }; using mint = ModInt; typedef vector> Matrix; Matrix mul(Matrix a, Matrix b) { assert(a[0].size() == b.size()); int i, j, k; int n = a.size(), m = b[0].size(), l = a[0].size(); Matrix c(n, vector(m)); for (i = 0; i < n; i++) for (k = 0; k < l; k++) for (j = 0; j < m; j++) c[i][j] += a[i][k] * b[k][j]; return c; } Matrix mat_pow(Matrix x, ll n) { ll k = x.size(); Matrix ans(k, vector(k, 0)); for (int i = 0; i < k; i++) ans[i][i] = 1; while (n != 0) { if (n & 1) ans = mul(ans, x); x = mul(x, x); n = n >> 1; } return ans; } int main(){ ll n, m; cin >> n >> m; ll b[m]; rep(i, m) cin >> b[i]; double e[m], sum = 0; rep2(i,m){ if(i+1 == m) e[i] = n - b[i]; else e[i] = min((double)n - b[i], (double)m / (m - 1 - i) + sum / (m - 1 - i)); sum += e[i]; } cout << setprecision(10) << e[0] + b[0] - 1 << endl; }