#include<bits/stdc++.h>
#include<stdio.h>
#include<stdlib.h>

using namespace std;
#define endl "\n"
#define FILEIO freopen("input.txt", "r", stdin);freopen("output.txt", "w", stdout);
#define LL long long 
#define mod 1000000007LL
#define fast ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define pb push_back
#define pp pop_back
//#define p push
#define I insert
#define bg begin()
#define ed end()
#define sz size()
#define all(v) v.begin(),v.end()
typedef vector<LL> vi;
typedef pair<int,int> ii;
typedef vector<ii> vii;
typedef map<LL ,LL > mii;
//typedef priority_queue<int> pq;
typedef queue<int>que;
typedef stack<int>stk;
#define Inf 100000005LL
#define f first
#define s second






int main(){

  
	  double n;
	  cin>>n;
	  cout<< 2 * 3.14*n;

   
}