#include <bits/stdc++.h>
#include <iostream>
#include <numeric>
#include <string>
#include <list>
#include <limits>
#include <type_traits>

using namespace std;

int gcd(int a, int b)
{
    if (a % b == 0)
    {
        return(b);
    }
    else
    {
        return(gcd(b, a % b));
    }
}


int main(void) {

    double k;
    double p = 3.141592653589793238462643383279;
    
    cin >> k;

    k = 2 * k * p;
    
    cout << k;
}