#include <bits/stdc++.h>
#include <string>
#include <iostream>
#include <iomanip>
#include <cstdlib> //絶対値整数abs
#include <cmath>   //絶対値少数fbs
#include <math.h>
using namespace std;
#define rep(i, N) for (ll i = 0; i < (N); i++)
typedef long long ll;

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll R;
    cin >> R;
    cout << R * 3.1415 * 2 << endl;
}