///Bismillahir Rahmanir Rahim
///Hello_World_Its_Rakib_Alam_Bappi
#include<bits/stdc++.h>
#include <cmath>
using namespace std;
///ascii for  A=65   and   a=97
#define		int 	long long
#define 		pb 	push_back
#define 		mp 	make_pair
#define 		pi 	2*acos(0.0)
#define 		loopi 	for(int i = 0 ; i <t ; i++)
#define 		loopj 	for(int j = 0 ; j < n ; j++)
#define 		backj 	for(int j = n-1 ; j >=0 ; j--)
#define 		vect 	vector<int>
#define 		vec_it 	vector<int>::iterator vi
#define 		mapp 	map<int,int>m
#define 		pairr 	pair<int,int>p
#define 		sett	set<int>s
#define 		set_it 	set<int>::iterator si
#define 		yes 	cout<<"YES"<<endl
#define 		no 	cout<<"NO"<<endl
#define 		xx 	first
#define 		xy 	second
main()
{
    /// freopen ("in.txt" , "r",  stdin) ;
    string st;
    double n,k,t,temp=0,cnt=0,x,y=0,sum=0;
    cin>>n;
    sum=2*M_PI*n;
    printf("%lf",sum);
    
}