#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
bool chmax(T& max, const U& x) {
  if (max < x) {
    max = x;
    return true;
  }
  return false;
}
template <typename T, typename U>
bool chmin(T& min, const U& x) {
  if (min > x) {
    min = x;
    return true;
  }
  return false;
}
int main() {
  double r;
  cin >> r;
  cout << 2 * r * M_PI << endl;
  cout << fixed << setprecision(8);
}
