#include <bits/stdc++.h>
using namespace std;
class Solution {
 public:
  void solve() {
    double r;
    cin >> r;
    cout << r * 3.141592 << "\n";
    return;
  };
};
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  Solution solution;
  solution.solve();
  return 0;
}
