#include<iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <math.h>
#include <functional>
#include <iomanip>
#include <bitset>
using namespace std;
#define rep(i, n) for (int i = 0; i < n; i++)
const long long mod = 1000000007;
typedef long long ll;

int main() {

	long double r;
	cin >> r;

	cout << long double(2 * r * PI) << endl;

	return 0;
}
