#include <stdio.h>
#include <vector>
#include <iostream>
#include <queue>
#include <bits/stdc++.h>
 
#define rep(i,n) for (int i=0;i<(n);++i)
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<double> vd;
typedef pair<int, int> pi;

const double PI=3.14159265358979323846;
int main() {
  double r;
  cin >> r;
  printf("%.6lf\n", 2 * r * PI);
  return 0;
}