#include <bits/stdc++.h>
#define ll long long
#define pii pair<int,int>
#define pll pair<ll,ll>
#define vi vector<int>
#define vl vetcor<ll>
#define vii vector<pii>
#define vll vector<pll>
#define IOS ios_base::sync_with_stdio(false)
using namespace std;
double long pi = 3.14159265;
int main()
{
	double r;
	cin>>r;
	cout<<fixed<<setprecision(10);
	cout<<2 * r * pi<<endl;
}