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

#define ll long long int
#define pb push_back
#define fi first
#define se second
#define mp make_pair
#define mt make_tuple
#define all(A) A.begin(), A.end()
#define sz(A) (int)A.size()
typedef vector <int> vi;
typedef vector <ll> vl;
typedef pair <int, int> pii;
const ll mod = 1e9 + 7;
//const ll mod = 998244353;
const int inf = 1e9;
//const ll inf = 1e18;
const int N = 2e5;

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	long double pi = 3.14159265358979323846;
	long double r;
	cin >> r;
	cout << 2 * pi * r << "\n";
 	return 0;
}