#include<bits/stdc++.h>
using namespace std;
#define very_slow       ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define pb              push_back
#define mp              make_pair
#define pii             pair<int,int>
#define vec             vector<int>
#define mii             map<int,int>
#define pqb             priority_queue<int>
#define pqs             priority_queue<int,vi,greater<int> >
#define inf             1e18
#define MAX             1e9
#define MIN            -1e9
#define no_of_test(x)        int x; cin>>x; while(x--)
const double pi=3.14159265359;
//Actual Code Is At The Top
const int mod = 1e9 + 6;
int gcd(int a, int b)
{
	if (b == 0) return a;
	return gcd(b, a % b);
}
int binary_exponentation(int a, int b)
{
	int ret = 1;
	while (b)
	{
		if (b & 1) ret = (ret * a) % mod;
		b >>= 1;
		a = (a * a) % mod;
	}
	return ret;
}
int main()
{   
	very_slow
//	no_of_test(x)
	{  
       double r;
	   cin>>r;
	   double k=(2.0)*pi*r;
        cout<<fixed<<setprecision(16)<<k<<'\n';
	}
	return 0;
}
#include <bits/stdc++.h>
#define rep(i,n) for(long long i=0; i<n; i++)
#define Rep(i,n) for(long long i=1; i<n; i++)
#define ll long long
#include <math.h>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <queue>
#include <set>
#include <vector>


using namespace std;
#define PI acos(-1)




int main(){
    ll N;
    cin>>N;
  
  cout<<2*N*PI<<endl;
  return 0;
}

