#include<bits/stdc++.h>

using namespace std;

int main() {

	// freopen("input.txt", "r", stdin);
	// freopen("output.txt", "w", stdout);

	double r;
	const double PI = 3.141592653589793;
	cin >> r;
	cout << 2 * PI * r << '\n';

	return 0;
}