/*-- ILSH  --*/
#include "bits/stdc++.h"
using namespace std;
typedef long long int ll;
#define pb push_back 
#define pop pop_back
#define mp make_pair
#define vii vector < int >
#define vll vector < ll > 
#define pii pair < int , int >
#define pll pair < ll , ll >
#define vpi vector < pii >
#define vpl vector < pll >
#define fi first
#define sz size 
#define len length 
#define se second 

const ll mod = 1e9+7;
const ll inf=1e18+1e17;
//const ll mod = 998244353;

ll modInverse(ll a,ll m){ll m0=m;ll y=0,x=1;if(m == 1)return 0;while(a> 1){ll q=a/m;ll t=m;m=a%m,a=t;t=y;y=x-q*y;x=t;}if(x<0)x+=m0;return x;}  
ll powm(ll a,ll b){a=a%mod;ll res=1;while(b){if(b&1)res=(res*a)%mod;a=(a*a)%mod;b>>=1;}return (res%mod);}

const ll N=1e5+6;
const long double pi=3.14159265358979323;

void solve(){


	long double r ;
	cin>>r;
	r=2*pi*r;
	cout<<setprecision(6)<<fixed;
	cout<<r<<'\n';







}


int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
    ll t=1;
//	cin>>t;
    for (int i=1;i<=t;i++){
    //	cout<<"Case #"<<i<<": ";
    	solve();
 	}
}



