#include<iostream>
#include<bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define ll long long
#define N 2*10e5
#define pi  3.141592
using namespace std;
void get(ll arr[],ll n){for(ll i=0;i<n;i++) cin>>arr[i];}
int main()
{
 	ios::sync_with_stdio(0);
	cin.tie(0);
	ll r;
	cin>>r;
	float c = 2*pi*r;
	cout<<fixed<<setprecision(3)<<c;
}

