/*********বিসমিল্লাহির রাহমানির রাহিম**********/

#include <bits/stdc++.h>
#define ll long long
#define PI 2 * acos(0)
#define FAST ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#define MAX 200000
#define MOD 1e7
#define all(x) (x).begin(), (x).end()
using namespace std;
/*................OMI.................*/

vector<ll> prime;
bool isPrime(ll n)
{
   if (n <= 1)
      return false;
   if (n <= 3)
      return true;

   if (n % 2 == 0 || n % 3 == 0)
      return false;

   for (ll i = 5; i * i <= n; i = i + 2)
      if (n % i == 0)
         return false;

   return true;
}
ll gcd(ll a, ll b)
{
   if (a == 0)
      return b;
   return gcd(b % a, a);
}

ll lcm(ll a, ll b)
{
   ll g = gcd(a, b);
   ll ans = (a * b) / g;
   return ans;
}
void primes(ll m)
{
   bool isprime[m];

   for (int i = 0; i <= m; i++)
      isprime[i] = true;

   for (int i = 3; i * i <= m; i++)
   {
      if (isprime[i])
      {

         for (int j = i * i; j <= m; j += i)
            isprime[j] = false;
      }
   }
   prime.push_back(2);
   for (int i = 3; i <= m; i += 2)
      if (isprime[i])
         prime.push_back(i);
}

/************MAIN_FUNCTION***********/

int main(int argc, char *argv[])
{

   FAST;
   double r;

   cin>>r;

   cout<<setprecision(23);
   cout<<2*PI*r<<endl;

   return 0;
}