#include<bits/stdc++.h>
#define mp make_pair
#define pr pair<ll,ll>
#define MAX 1000000
#define ed '\n'
using namespace std;
typedef long long int ll;
#define mod 998244353
double pi=3.14159265358979323846;
void sync(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
}

int main(){
	sync();
	ll t;
	t=1;
	for(ll testcase=1;testcase<=t;testcase++){
	ll r;
	cin>>r;
	double area=2*pi*r;
	cout<<fixed<<setprecision(20)<<area<<endl;
	}
}