#include <iostream>
#include <list>
#include <iterator>
#include <algorithm>
#include <bits/stdc++.h>
#include <bitset>
#include <set>
#include <stdio.h>
#include <cmath>
 
#define loop(i,n)   for(i=0;i<n;i++)
#define ll          long long
#define MAX         10000001
#define pi          acos(-1)

ll i,j,k,cnt,sum;
 
using namespace std;

bitset<MAX> prime;

void seive(){
	prime[0] = 1;
	prime[1] = 1;
	prime[2] = 0;

	for(i=4;i<MAX;i+=2){
		prime[i] = 1;
	}

	for(i=3;i*i<=MAX;i+=2){
		if(!prime[i]){
			for(j=i+i;j<MAX;j+=i){
				prime[j] = 1;
			}
		}
	}
}


//Code Starts Here//
int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);

	int r;
	cin>>r;

	cout<<r*2*pi<<endl;


	return 0;
}
