#define __USE_MINGW_ANSI_STDIO
#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
#include <unordered_set>
#include <unordered_map>

using namespace std;

typedef long long ll;
typedef long long ull;
typedef pair<int, int> ii;

#define all(v)		((v).begin()), ((v).end())
#define sz(v)		((int)((v).size()))
#define endl		"\n"
#define fx(n)		fixed<<setprecision(n)
#define mk			make_pair	

void File()
{
#ifndef ONLINE_JUDGE
	freopen("output.txt", "w", stdout);
	freopen("input.txt", "r", stdin);
#endif
#ifdef ONLINE_JUDGE
	/*freopen("output.txt", "w", stdout);
	freopen("pyramid.in", "r", stdin);*/
#endif
}
void fast()
{
	ios::sync_with_stdio(NULL);
	cout.tie(NULL);
	cin.tie(NULL);
	//File();
}
const double pi = 2 * acos(0.0);
const ll oo = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
const int nn = 2e5 + 15;
int dx[8] = { 1, -1, 0, 0, 1, -1, 1, -1 };
int dy[8] = { 0, 0, 1, -1, 1, -1, -1, 1 };

int main()
{
	fast();
	double b;
	cin >> b;
	cout << fx(20) << 2 * pi*b << endl;
	return 0;
}