#include <stdio.h>
#include <iostream>
#include <cstdlib>
#include <vector>
#include <queue>
#include <math.h>
#include <string> 
#include <bits/stdc++.h>
using namespace std;

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