#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
const int N = (int)1e6 + 100;
const int M = (int)2e6 + 100;
const int inf = (int)1e9 + 100;
int main() {
  mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
  srand(time(0));
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long double a;
  cin >> a;
  cout << setprecision(60) << 2 * a * 3.14159265358979323846;
}
