//#define _USE_MATH_DEFINES
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <vector>
using namespace std;

#define PI = 3.1415

int main() {
	int r; cin >> r;
	cout << 2 * r * 3.141592 << endl;
	return 0;
}