#define _USE_MATH_DEFINES
#include<bits/stdc++.h>
#include<unordered_set>
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define LSB(i) ((i) & -(i))
#define MAXSIZE 100000000

using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;

int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
int lcm(int a, int b) { return a * (b / gcd(a, b)); }

int main() {

	ios::sync_with_stdio(0); cin.tie(0);

	//freopen("in.txt", "r", stdin);
	//freopen("out.txt", "w", stdout);

	double n; cin >> n;
	cout << (n * 2) * M_PI << "\n";
}
