Language: C

#include <bits/stdc++.h>
using namespace std;
#define ll long long
//#define ll unsigned long long
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define be begin()
#define en end()
#define le length()
#define all(x) (x).begin(), (x).end()
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repo(i, n) for (ll i = 1; i <= n; i++)
#define foreach(c, itr) for (__typeof((c).begin()) itr = (c).begin(); itr != (c).end(); itr++)
#define fastIO                        \
	ios_base::sync_with_stdio(false); \
	cin.tie(NULL);                    \
	cout.tie(NULL)
#define eps 1e-6
#define pi 3.141592653589793
#define c(a) cout << a << "\n";
#define ao(a)        \
	for (auto i : a) \
		cout << i << " ";
#define endl '\n'
typedef vector<int> vii;
typedef vector<ll> vll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int MOD = 1e9 + 7;
const int INF = 1e9;
const int N = 1e6 + 6;

ll powmod(ll a, ll b, ll mod)
{
	ll res = 1;
	a %= mod;
	for (; b; b >>= 1)
	{
		if (b & 1)
			res = (res * a) % mod;
		a = (a * a) % mod;
	}
	return res;
}

ll gcd(ll a, ll b)
{
	while (b > 0)
	{
		ll t = a % b;
		a = b, b = t;
	}
	return a;
}

ll lcm(ll a, ll b)
{
	return (a / gcd(a, b)) * b;
}

ll is_prime(ll n)
{
	if (n <= 1 || n > 3 && (n % 2 == 0 || n % 3 == 0))
		return 0;
	for (ll i = 5, t = 2; i * i <= n; i += t, t = 6 - t)
		if (n % i == 0)
			return 0;
	return 1;
}
ll modI(ll a, ll m)
{
	ll m0 = m, y = 0, x = 1;
	if (m == 1)
		return 0;
	while (a > 1)
	{
		ll q = a / m;
		ll t = m;
		m = a % m;
		a = t;
		t = y;
		y = x - q * y;
		x = t;
	}
	if (x < 0)
		x += m0;
	return x;
}
// ll powbin(ll base, ll p)
// {
// 	ll mul;

// 	if (p == 1) mul = base;
// 	else {
// 		mul = (ll)powbin(base, p / 2);
// 		mul = (mul * mul) % MOD;
// 		if (p % 2) mul *= base;
// 	}

// 	return mul % MOD;
// }

// ll combination(ll n, ll r)
// {
// 	ll num = 1, den = 1;

// 	for (ll i = n; i >= n - r + 1; i--) {
// 		num = (num * i) % MOD;
// 	}
// 	for (ll i = r; i >= 1; i--) {
// 		den = (den * i) % MOD;
// 	}
// 	return (num * powbin(den, MOD - 2)) % MOD;
// }

// int v[(1<<20)];

// void init(void){
//     int i;
//     rep(i,(1<<20)) v[i] = 0;
// }

// void add(int x, int val){
//     for(int i=x;i<(1<<20);i=((i)|(i+1))) v[i] += val;
// }

// int sum(int x){
//     ll ans = 0;
//     for(int i=x;i>0;i=((i)&(i-1))) ans += v[i-1];
//     return ans;
// }
// long long get(vector<long long> &f, int pos) {
// 	long long res = 0;
// 	for (; pos >= 0; pos = (pos & (pos + 1)) - 1)
// 		res += f[pos];
// 	return res;
// }

// void upd(vector<long long> &f, int pos, int val) {
// 	for (; pos < int(f.size()); pos |= pos + 1) {
// 		f[pos] += val;
// 	}
// }
/*************************************************************************************************************/
// int tree[N],a[N];
// ll build(ll l,ll r,ll node){
//     if(r<l) return N;
//     if(l==r) {tree[node]=a[l];return N;}
//     ll mid=(l+r)/2;
//     build(l,mid,2*node+1);
//     build(mid+1,r,2*node+2);
//     tree[node] = min(tree[2*node+1],tree[2*node+2]);

// }
// void update(ll l,ll r,ll node,ll val,ll pos){
//     if(r<l) return;
//     if(l==r && l==pos) {tree[node]=val;return;}
//     ll mid=(l+r)/2;
//     if(pos<=mid) update(l,mid,2*node+1,val,pos);
//     else update(mid+1,r,2*node+2,val,pos);
//     tree[node]=min(tree[2*node+1],tree[2*node+2]);
// }
// ll sum(ll l,ll r,ll x,ll y,ll node){
//     //if(x<l || y>r) return 0;
//     if(r<x || y<l) return N;
//     //if(l==r) return tree[node];
//    // if(y<x || r<l) return 0;
//     if(x<=l && r<=y) return tree[node];
//     ll mid=(l+r)/2;
//     return min(sum(l,mid,x,y,2*node+1),sum(mid+1,r,x,y,2*node+2));
// }
// bool prime[100005];
// void SieveOfEratosthenes(int n)
// {

//     memset(prime, true, sizeof(prime));

//     for (int p=2; p*p<100005; p++)
//     {

//         if (prime[p] == true)
//         {

//             for (int i=p*p; i<100005; i += p)
//                 prime[i] = false;
//         }
//     }
// }
int main()
{
	fastIO;
	double r;
	cin >> r;
	cout << fixed << setprecision(8) << 2 * pi * r;

	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.