// n using Sieve of Eratosthenes 
#include <bits/stdc++.h> 
#include <iostream>
#include <cmath>
using namespace std;
# define M_PI 3.14159265358979323846 

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