Language: C

#include <bits/stdc++.h>
const long long INF = 1LL << 60;
const long long MOD = 1000000007;
#define rep(i, n) for (ll i = 0; i < (n); ++i)
#define rep1(i, n) for (ll i = 1; i <= (n); ++i)
#define rrep(i, n) for (ll i = (n - 1); i >= 0; --i)
#define perm(c)   \
    sort(ALL(c)); \
    for (bool c##p = 1; c##p; c##p = next_permutation(ALL(c)))
#define ALL(obj) (obj).begin(), (obj).end()
#define RALL(obj) (obj).rbegin(), (obj).rend()
#define pb push_back
#define to_s to_string
#define len(v) (ll) v.size()
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define print(x) cout << (x) << '\n'
#define drop(x) cout << (x) << '\n', exit(0)
#define debug(x) cout << #x << ": " << (x) << '\n'
using namespace std;
using ll = long long;
typedef pair<ll, ll> P;
typedef vector<ll> vec;
typedef vector<vector<ll>> vec2;
typedef vector<vector<vector<ll>>> vec3;
template <class S, class T>
inline bool chmax(S &a, const T &b)
{
    if (a < b)
    {
        a = b;
        return 1;
    }
    return 0;
}
template <class S, class T>
inline bool chmin(S &a, const T &b)
{
    if (b < a)
    {
        a = b;
        return 1;
    }
    return 0;
}
inline ll msb(ll v) { return 1 << (31 - __builtin_clzll(v)); }
inline ll devc(ll x, ll y) { return (x + y - 1) / y; }

struct IoSetup
{
    IoSetup()
    {
        cin.tie(nullptr);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(10);
        cerr << fixed << setprecision(10);
    }
} iosetup;

template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p)
{
    os << p.first << " " << p.second;
    return os;
}

template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p)
{
    is >> p.first >> p.second;
    return is;
}

template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v)
{
    for (int i = 0; i < (int)v.size(); ++i)
    {
        os << v[i] << (i + 1 != v.size() ? " " : "");
    }
    return os;
}

template <typename T>
istream &operator>>(istream &is, vector<T> &v)
{
    for (T &in : v)
        is >> in;
    return is;
}

/*---------------------------------    Tools    ------------------------------------------*/
template <typename T>
vector<T> cumsum(const vector<T> &X)
{
    vector<T> res(X.size() + 1, 0);
    for (int i = 0; i < X.size(); ++i)
        res[i + 1] += res[i] + X[i];
    return res;
}

//f :　単調性の関数
template <typename S, typename T, typename F>
pair<T, T> bisearch(S left, T right, F f)
{
    while (abs(right - left) > 1)
    {
        T mid = (right + left) / 2;
        if (f(mid))
            right = mid;
        else
            left = mid;
    }
    return {left, right};
}

template <typename S, typename T, typename F>
double trisearch(S left, T right, F f, double precision = 1e-10)
{
    double low = left, high = right;
    while (high - low > precision)
    {
        double mid_left = high / 3 + low * 2 / 3;
        double mid_right = high * 2 / 3 + low / 3;
        if (f(mid_left) >= f(mid_right))
            low = mid_left;
        else
            high = mid_right;
    }
    return high;
}

/*------------------------------- Main Code Here -----------------------------------------*/

int main()
{
    double R;
    cin >> R;
    drop(2 * R * 3.14159265);

    return 0;
}

Compiler output:
main.c:1:10: fatal error: bits/stdc++.h: No such file or directory
    1 | #include <bits/stdc++.h>
      |          ^~~~~~~~~~~~~~~
compilation terminated.