#include <bits/stdc++.h>//復習必須
#include<iostream>
#include<vector>
#include <cmath>
#include <map>
#include <algorithm>
#include <string>
#define rep(i, n) for (int i = 0; i < n; ++i)
using ll = long long;
using namespace std;
#define P pair<int, int>
int main () {
    double r ;
    cin >> r ;
    const double PI=3.14159265358979323846;

    cout << 2 * PI * r << endl;
    return 0 ;
}

 
