#include <bits/stdc++.h>
using namespace std;
template <class t, class u>
void chmax(t& a, u b) {
  if (a < b) a = b;
}
template <class t, class u>
void chmin(t& a, u b) {
  if (b < a) a = b;
}
template <class t>
using vc = vector<t>;
template <class t>
using vvc = vc<vc<t>>;
using pi = pair<int, int>;
using vi = vc<int>;
void yes() { cout << "Yes" << endl; }
void no() { cout << "No" << endl; }
int SIZE(string s) { return (int)s.size(); }
int main() {
  cout << fixed << setprecision(20);
  int r;
  cin >> r;
  r *= 2;
  cout << r * M_PI << endl;
  return 0;
}
