Language: C++

#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#include<set>
#include<queue>
#include<stack>
#include<bitset>
#include<functional>
#include<map>
#include<iomanip>
#include<limits>
#include<unordered_set> 
#include<cmath>
#include <numeric>
#include <array>
#include <complex>
double M_PI = 3.1415926535897932384626;
using namespace std;
//long long p = 998244353;
long long p = 1000000007;
#define int long long
#define vel vector<long long>
#define vvel vector<vel>
#define rep(i,n) for(int i=0;i<n;i++)
#define sor(v) sort(v.begin(),v.end())
#define mmax(a,b) a=max(a,b)
#define mmin(a,b) a=min(a,b)
#define mkp make_pair
#define pin pair<int,int>
#define qin pair<pin,int>
#define V vector
#define Endl endl
#define veb vector<bool>
#define fcout cout << fixed << setprecision(15)
#define rev(s) reverse(s.begin(),s.end())
#define lower(h,val) lower_bound(h.begin(),h.end(),val)-h.begin()
#define upper(h,val) upper_bound(h.begin(),h.end(),val)-h.begin()
int max_kai = 10000;
vel kai(max_kai, 1);
vel inv_kai(max_kai, 1);
int rui(int a, int n, int mod) {
    if (n == 0) { return 1 % mod; }
    int x = rui(a, n / 2, mod);
    x *= x; x %= mod;
    if (n % 2 == 1) { x *= a; x %= mod; }
    return x;
}
int root(int x,vel &pa) {
    if (pa[x] == -1) { return x; }
    int ans = root(pa[x],pa); pa[x] = ans;
    return ans;
}
bool mar(int x, int y,vel &pa) {
    x = root(x,pa);
    y = root(y,pa);
    if (x != y) { pa[x] = y; }
    return (x != y);
}
int gcd(int x, int y) {
    if (x < y) { return gcd(y, x); }
    if (y == 0) { return x; }
    return gcd(y, x % y);
}
long long modinv(long long a, long long m) {
    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;
}
void make_kai() {
    rep(i, max_kai - 1) {
        kai[i + 1] = kai[i] * (i + 1); kai[i + 1] %= p;
        inv_kai[i + 1] = modinv(kai[i + 1], p);
    }
}
int com(int n, int r) {
    if ((n < 0) || (r < 0) || (r > n)) { return 0; }
    int ans = (kai[n] * inv_kai[r]) % p;
    return (ans * inv_kai[n - r]) % p;
}
vel uni(vel x) {
    if (x.size() == 0) { return x; }
    sor(x);
    int n = x.size();
    vel ans(1, x[0]);
    for (int j = 1; j < n; j++) {
        if (x[j - 1] != x[j]) { ans.push_back(x[j]); }
    }
    x = ans;
    return x;
}
void pr(vel& v) {
    int n = v.size();
    if (n != 0) {
        cout << v[0];
        rep(i, n - 1) {
            cout << " " << v[i + 1];
        }
        cout << endl;
    }
}
int inf = 100000;
vel dijk(V<V<pin>>& way, int st) {
    int n = way.size();
    vel dist(n, inf); dist[st] = 0;
    priority_queue<pin, vector<pin>, greater<pin>> pq;
    pq.push(mkp(0, st));
    veb is_checked(n, false);
    while (!pq.empty()) {
        pin x = pq.top(); pq.pop();
        int pot = x.second;
        if (!is_checked[pot]) {
            is_checked[pot] = true;
            for (auto y : way[pot]) {
                int nex_dist = x.first + y.second;
                int nex_pot = y.first;
                if (dist[nex_pot] > nex_dist) {
                    dist[nex_pot] = nex_dist;
                    pq.push(mkp(nex_dist, y.first));
                }
            }
        }
    }
    return dist;
}
template< typename Monoid >
struct SegmentTree {
    using F = function< Monoid(Monoid, Monoid) >;

    int sz;
    vector< Monoid > seg;

    const F f;
    const Monoid M1;

    SegmentTree(int n, const F f, const Monoid& M1) : f(f), M1(M1) {
        sz = 1;
        while (sz < n) sz <<= 1;
        seg.assign(2 * sz, M1);
    }

    void set(int k, const Monoid& x) {
        seg[k + sz] = x;
    }

    void build() {
        for (int k = sz - 1; k > 0; k--) {
            seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
        }
    }

    void update(int k, const Monoid& x) {
        k += sz;
        seg[k] = x;
        while (k >>= 1) {
            seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
        }
    }

    Monoid query(int a, int b) {
        Monoid L = M1, R = M1;
        for (a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
            if (a & 1) L = f(L, seg[a++]);
            if (b & 1) R = f(seg[--b], R);
        }
        return f(L, R);
    }

    Monoid operator[](const int& k) const {
        return seg[k + sz];
    }

    template< typename C >
    int find_subtree(int a, const C& check, Monoid& M, bool type) {
        while (a < sz) {
            Monoid nxt = type ? f(seg[2 * a + type], M) : f(M, seg[2 * a + type]);
            if (check(nxt)) a = 2 * a + type;
            else M = nxt, a = 2 * a + 1 - type;
        }
        return a - sz;
    }


    template< typename C >
    int find_first(int a, const C& check) {
        Monoid L = M1;
        if (a <= 0) {
            if (check(f(L, seg[1]))) return find_subtree(1, check, L, false);
            return -1;
        }
        int b = sz;
        for (a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
            if (a & 1) {
                Monoid nxt = f(L, seg[a]);
                if (check(nxt)) return find_subtree(a, check, L, false);
                L = nxt;
                ++a;
            }
        }
        return -1;
    }

    template< typename C >
    int find_last(int b, const C& check) {
        Monoid R = M1;
        if (b >= sz) {
            if (check(f(seg[1], R))) return find_subtree(1, check, R, true);
            return -1;
        }
        int a = sz;
        for (b += sz; a < b; a >>= 1, b >>= 1) {
            if (b & 1) {
                Monoid nxt = f(seg[--b], R);
                if (check(nxt)) return find_subtree(b, check, R, true);
                R = nxt;
            }
        }
        return -1;
    }
};
vel mul(vel& a, vel& b) {
    int n = a.size();
    int m = b.size();
    vel ans(n + m - 1, 0);
    rep(i, n) {
        rep(j, m) {
            ans[i + j] += a[i] * b[j];
            ans[i + j] %= p;
        }
    }
    return ans;
}
vel rui_p(vel& a, int n) {
    if (n == 0) { return{ 1 }; }
    vel qans = rui_p(a, n / 2);
    qans = mul(qans, qans);
    if (n % 2 == 1) {
        qans = mul(qans, a);
    }
    return qans;
}
int solve(vel& h) {
    int n = h.size();
    if (n == 0) { return 0; }
    if (n == 1) { return h[0]; }
    int mn = h[0];
    rep(i, n) { mmin(mn, h[i]); }
    rep(i, n) { h[i] -= mn; }
    int ans = mn;
    vel v;
    rep(i, n) {
        if (h[i] == 0) { ans += solve(v); v = {}; }
        else {
            v.push_back(h[i]);
        }
    }
    ans += solve(v);
    return ans;
}
// Cooley–Tukey FFT algorithm O(N log N)
vector<complex<double>> fft(vector<complex<double>> a, bool inverse = false) {
    int n = a.size();
    int h = 0; // h = log_2(n)
    for (int i = 0; 1 << i < n; i++) h++;
    // バタフライ演算用の配置入れ替え
    for (int i = 0; i < n; i++) {
        int j = 0;
        for (int k = 0; k < h; k++) j |= (i >> k & 1) << (h - 1 - k);
        if (i < j) swap(a[i], a[j]);
    }
    // バタフライ演算
    for (int b = 1; b < n; b *= 2) {
        // 第 log_2(b) + 1 段
        // ブロックサイズ = b * 2
        for (int j = 0; j < b; j++) {
            // ブロック内 j 個目
            // 重み w = (1 の原始 2b 乗根の j 乗)
            complex<double> w =
                polar(1.0, (2 * M_PI) / (2 * b) * j * (inverse ? 1 : -1));
            for (int k = 0; k < n; k += b * 2) {
                // k を先頭とするブロック
                complex<double> s = a[j + k];         // 前
                complex<double> t = a[j + k + b] * w; // 後
                a[j + k] = s + t;                     // 前の更新
                a[j + k + b] = s - t;                 // 後の更新
            }
        }
    }
    // 逆変換時にサイズで割る調整
    if (inverse)
        for (int i = 0; i < n; i++) a[i] /= n;
    return a;
}
// Cooley–Tukey FFT algorithm O(N log N)
vector<complex<double>> fft(vector<double> a, bool inverse = false) {
    vector<complex<double>> a_complex(a.size());
    for (int i = 0; i < a.size(); i++) a_complex[i] = complex<double>(a[i], 0);
    return fft(a_complex, inverse);
}

// FFT による畳み込み O(N log N)
vector<double> convolve(vector<double> a, vector<double> b) {
    int s = a.size() + b.size() - 1; // 畳み込み結果のサイズ
    int t = 1; // FFT に使う配列のサイズ（2 の累乗）
    while (t < s) t *= 2;
    a.resize(t); // FFT するためにリサイズ
    b.resize(t); // FFT するためにリサイズ
    vector<complex<double>> A = fft(a);
    vector<complex<double>> B = fft(b);
    for (int i = 0; i < t; i++) {
        A[i] *= B[i]; // 畳み込み結果の FFT 結果を得る
    }
    A = fft(A, true); // IFFT で畳み込み結果を得る
    a.resize(s);      // 畳み込み結果を入れるためにリサイズ
    for (int i = 0; i < s; i++) a[i] = A[i].real(); // 実部が答え
    return a;
}
signed main(){
    int n; cin >> n;
    V<double> a(n+1, 0);
    double now_sum = 0;
    for(int i=1;i<=n;i++) {
        now_sum += a[i - 1];
        double mi = i;
        a[i] = 2*now_sum / mi;
        a[i] += (mi + 1) / 2;
    }
    double d1, x; cin >> d1 >> x;
    d1 += ((2 * n - 1) * x) / 2;
    fcout << a[n] * d1 << endl;
    return 0;
}


Compiler output:
In file included from /usr/include/c++/14/cmath:47,
                 from a.cc:14:
a.cc:18:8: error: expected unqualified-id before numeric constant
   18 | double M_PI = 3.1415926535897932384626;
      |        ^~~~