#include <bits/stdc++.h>
using namespace std;
const double PI = 3.14159265358979323846;
const double EPS = 1e-9;
const long long INF = 10e17;
double equal(double a, double b) { return fabs(a - b) < DBL_EPSILON; }
std::istream &operator>>(std::istream &in, set<int> &o) {
  long long a;
  in >> a;
  o.insert(a);
  return in;
}
std::istream &operator>>(std::istream &in, queue<int> &o) {
  long long a;
  in >> a;
  o.push(a);
  return in;
}
bool contain(set<int> &s, int a) { return s.find(a) != s.end(); }
const int mod = 1000000007;
int main() {
  double r;
  cin >> r;
  printf("%.20f\n", r * 2 * PI);
}
