#include <bits/stdc++.h>
using namespace std;
template <typename T>
ostream& operator<<(ostream& out, vector<T>& v) {
  for (int i = 0; i < (int)v.size(); i++) {
    out << v[i] << ' ';
  }
  return out;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  float n;
  cin >> n;
  cout << n * 2 * 3.141592653589 << endl;
  return 0;
}
