#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef vector<ll> vll;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define Sort(a) sort(a.begin(), a.end())
#define fin(ans) cout << (ans) << endl
const int INF = 1<<30;
const ll MOD = 1000000007;

int main() {
  string s,t,u;
  double a, b;
  cin >> a;
  //cin >> a >> b;
  //cin >> s >> t >> u;
  
  cout << 2*a*M_PI << endl; 
  
}