#include<iostream>
#include<stdio.h>
#include<string>
#include<cmath>
#include<math.h>
#include<vector>
#include<algorithm>
#include<map>
#pragma warning(disable: 4996)
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
using ll = long long;


int main() {
	const double PI = 3.14159265358979323846;
	double R,a;
	cin >> R;

	a = R*2 * PI;

	cout << a << endl;

	return 0;
}