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

// in-short-use macros

#define all(c) (c).begin(),(c).end()
#define ll long long int
#define ld long double
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define mod 1000000007
#define PI 3.14159265
#define sizes 1000010

# define M_PI           3.14159265358979323846

//main function here

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    
    ld n;
    cin>>n;
    cout<<2*n*M_PI<<"\n";
    return 0;
}