#include <bits/stdc++.h>
template <class T>
bool chmin(T& a, const T& b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmax(T& a, const T& b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
using namespace std;
using ll = long long;
using P = pair<int, int>;
int main() {
  double pi = 3.141592653589793238;
  double r;
  cin >> r;
  printf("%.12f\n", r * 2 * pi);
  ;
  return 0;
}
