#include <bits/stdc++.h>
using namespace std;
const double TL = 1.5 * CLOCKS_PER_SEC;
mt19937 rng(
    (unsigned int)chrono::steady_clock::now().time_since_epoch().count());
const double pi = acos(-1.0);
int main() {
  double r;
  cin >> r;
  cout << 2 * pi * r << endl;
  return 0;
}
