
// Problem : A - Circle Pond
// Contest : AtCoder - AtCoder Beginner Contest 163
// URL : https://atcoder.jp/contests/abc163/tasks/abc163_a
// Memory Limit : 1024 MB
// Time Limit : 2000 ms
// Powered by CP Editor (https://github.com/cpeditor/cpeditor)


/**         srijan_de
 _____      _ _              ______     
/  ___|    (_|_)             |  _  \    
\ `--. _ __ _ _  __ _ _ __   | | | |___ 
 `--. \ '__| | |/ _` | '_ \  | | | / _ \
/\__/ / |  | | | (_| | | | | | |/ /  __/
\____/|_|  |_| |\__,_|_| |_| |___/ \___|
            _/ |                         
           |__/           23.02.2001     **/
///.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.
#include    <bits/stdc++.h>  
using       namespace       std;
using       lli=            long long  int;
using       ulli=           unsigned long long int;
using       ld=             long double;
#define     mem(a,val)      memset( a,  val,  sizeof(a) )
#define     b_s             binary_search
#define     u_b             upper_bound
#define     l_b             lower_bound
#define     nl              cout<<endl
#define     pb              push_back
#define     pf              push_front
#define     ppb             pop_back
#define     ppf             pop_front
#define     in              insert
#define     fi              first
#define     se              second
#define 	pqueue			priority_queue
const lli   MOD=            (int)1e9 + 7;
const lli   MOD1=           (int)1e9 + 9;
const lli   MOD2=           998244353;
const ld    pi=             atan2(0, -1);
const ld    eps=            1e-9;
lli         tests=          1;
template<class x,class y>x max(x a,y b){return (a>b)?a:b;}
template<class x,class y>x min(x a,y b){return (a<b)?a:b;}
template<class x,class y>x log(x n,y r){return (n>r-1)?1+log(n/r,r):0;}

///.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.
int32_t main() 
{
	auto start = clock();
ios_base::sync_with_stdio(0);cout.tie(0);cin.tie(0);	

//	cout<<fixed;
//	cout<<scientific;
	cout<<fixed<<setprecision(20);
	// cin>>tests;
	for(lli test=1;test<=tests;test++) 
	{
//		cout<<"Case #"<<test<<": ";
///.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.
		
		
		
		lli n;
		cin>>n;
		cout<<double(2*n*pi);
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
		
///.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.
	}
	cerr<<endl<<endl<<endl<< "Time : " << double(clock() - start) / CLOCKS_PER_SEC <<endl;
	return 0;
}
///.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.'.
