#include<bits/stdc++.h>
#include<fstream>
using namespace std;
#define ll long long int 
#define endl '\n'
#define pb push_back
#define M 1000000007
#define sz(c) c.size()
#define mp make_pair
#define all(v) v.begin(),v.end()
#define rep(i,a,b) for(ll i=a;i<b;i++)
//const ll INF=2e18;

typedef vector<ll>   vi;
/*
bool isPrime(ll n){
	if(n<=1)
	return false;
	if(n<=3)
	return true;
	if(n%2==0 || n%3==0)
	return false;
	for(ll i=5;i*i<=n;i=i+6)
	{
		if(n%i==0 || n%(i+2)==0)
		return false;
	}
	return true;
}
*/
//bool sortbysec(const pair<char,ll> &a, const pair<char,ll> &b){
//	return (a.second<b.second);


void lucifer()
{
	ll n;
	cin>>n;
	cout<<2*(M_PI)*n;


}



signed main()
{
	ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);    
    ll t = 1;
    //clock_t start, end; 
    //start = clock(); 
 
    cout << fixed << setprecision(14);
   /* #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin); 
    freopen("output.txt", "w", stdout);
    freopen("error.txt", "w", stderr);
    #endif*/
   //cin>>t;
    while(t--){
        lucifer();
    }
 
    //end = clock();
    //double time_taken = double(end - start) / double(CLOCKS_PER_SEC); 
    //cerr<<time_taken;
}