#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <cmath>
#include <map>
#include <numeric>
#include <set>
#include <iomanip>
#include <functional>
#include <math.h>
#define FOR(i, a, n) for (long long i = (a); i < (n); i++)
#define REP(i, n) for(long long i = 0; i < (n); i++)
#define ALL(obj) (obj).begin(), (obj).end()
#define pb push_back
using namespace std;

typedef long long ll;

long long Mygcd(long long a, long long b)
{
	if (a % b == 0)return b;
	return Mygcd(b, a % b);
}

int Mygcd(int a, int b)
{
	if (a % b == 0)return b;
	return Mygcd(b, a % b);
}

long long Mylcm(long long a, long long b)
{
	return a * b / Mygcd(a, b);
}

int Mylcm(int a, int b)
{
	return a * b / Mygcd(a, b);
}

int main()
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	double R;

	cin >> R;

	double PI = 3.14159265358979323846;

	cout << std::fixed << std::setprecision(20) << R * 2 * PI << endl;

	return 0;
}