#include <iostream>
#include <iomanip>
#include <unordered_map>
#include <set>
#include <fstream>
#include <cstring>
#include <climits>
#include <algorithm>
#include <math.h>
#include <vector>
using namespace std;

#define ll long long
#define mod 1000000007

int main() {

	double r;
	cin >> r;
	cout << 2.0000 * 3.141592653589793238 * r;

	return 0;
}