#include<iostream>
#include <bits/stdc++.h>
#include <string>
#include <cstring>
#include <algorithm>
using namespace std;

const double PI=3.14159265358979323846;

int main() {
    int r;
    cin >> r ;
    
   cout << setprecision(10) << fixed << 2*PI << endl;
	return 0;
}

