#include<bits/stdc++.h>
using namespace std;

#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace __gnu_pbds; 

#define ll long long
#define db double
#define el "\n"
#define ld long double
#define rep(i,n) for(int i=0;i<n;i++)
#define rev(i,n) for(int i=n;i>=0;i--)
#define rep_a(i,a,n) for(int i=a;i<n;i++)
#define tr(ds, it) for(auto it=ds.begin(); it!=ds.end(); it++)
#define rtr(ds, it) for(auto it=ds.rbegin(); it!=ds.rend(); it--)
#define all(ds) ds.begin(), ds.end()
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
typedef priority_queue <int> pq;
typedef vector< long long > vi;
typedef pair<long long, long long> ii;
#define o_set tree<ll, null_type,less<ll>, rb_tree_tag,tree_order_statistics_node_update> 
#define graph vector<vi>

const ll MAX = 1000001;
const ll mod = 1000000007;
ll po(ll x, ll n ){ 
	ll ans=1;
     while(n>0){
     	if(n&1) ans=(ans*x)%mod;
     	x=(x*x)%mod;
     	n/=2;
     }
     return ans;
}

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);
	#ifndef ONLINE_JUDGE
	//reopen("input.txt", "r" , stdin);
	//freopen("output.txt", "w" , stdout);
	#endif

	ll r;
	cin>>r;
	
	cout<<setprecision(20)<<2*M_PI*r;

	return 0;
}
