#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<long long, long long>;
template <class T>
using vec = vector<T>;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(10);
  double r;
  cin >> r;
  ;
  cout << r * acos(-1) << '\n';
}
