#include <iostream>
#include <chrono>
#include <cmath>
#include <string.h>
#include <iomanip>
#include <algorithm>
#include <unordered_set>
#include <vector>
#include <queue>
#include <cmath>
#include <unordered_map>
#include <fstream>
#include <functional>

#define MAX(a, b) (a > b ? a : b)
#define MIN(a, b) (a < b ? a : b)

constexpr auto NMAX = (int)2e5 + 20;
constexpr auto INF = 2000000000;
using namespace std;

typedef unsigned long long ull;

int r;
double const pi = 3.14159;

int main()
{
	cin >> r;
	cout << 2 * pi * r << '\n';
	return 0;
}