#include <bits/stdc++.h>
#include <malloc.h>
#include <cstdlib>
using namespace std;
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tr_policy.hpp>
//using namespace __gnu_pbds;

#define ll				long long
#define double			long double
#define pb 				push_back
#define endl 			'\n'
#define F				first
#define S				second
#define mod				(int)1000000007
#define mp              make_pair
#define ios 			ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define inf				1e18
#define mem(a,x)		for(int i=0;i<(int)(sizeof(a)/sizeof(a[0]));i++) a[i] = x;
#define trace(a)		cout<<(#a)<<" --> "<<a<<endl
#define trace2(a,b)		{trace(a);trace(b);cout<<"------------------\n";}
#define trace3(a,b,c)	{trace(a);trace(b);trace(c);cout<<"------------------\n";}
#define trace4(a,b,c,d) {trace(a);trace(b);trace(c);trace(d);cout<<"------------------\n";}
#define M_PI           	3.14159265358979323846
#define T				pair<int,int>
//#define ordered_set 	tr<T, null_type,less<T>, rb_tr_tag,tr_order_statistics_node_update>


void run(){
	double r;
	cin>>r;
	cout<<(2*M_PI*r);
}

signed main(){
	ios;
//	int t;
//	cin>>t;
//	for(int i=1;i<=t;i++)
//		cout<<"Case #"<<i<<": ", run();
	return 0;
}
