#include <bits/stdc++.h>
using namespace std;
#define ll long long int
#define mp make_pair
#define pb push_back
#define pll pair<ll,ll>


int main()

{
	long double r;
	cin>>r;
	long double k = 3.141592653589;
	k = 2*r*k;
	cout<<k<<endl;
	return 0;
}