#include "bits/stdc++.h"

using namespace std;
using LL = long long;

constexpr int N = 1e5 + 5;

int main() {
	ios::sync_with_stdio(false); cin.tie(nullptr);
	
	int r;
	cin >> r;

	const double pi = acos(-1.0);

	cout << 2 * pi * r << endl;
}