#include<bits/stdc++.h>
#include<iostream>
#include <cmath>
using namespace std;

typedef long long ll;
typedef long double ld;
long double MAX = 10000009.0;

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    ld r;
    cin>>r;
    long double M_PI = 3.1415926535897;
     std::cout << std::setprecision(11) << 2*r*M_PI << endl;
    return 0;
}
