#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9 + 7;
signed main() {
  std::ios::sync_with_stdio(0);
  std::cin.tie(0);
  ;
  double a;
  cin >> a;
  cout << fixed << setprecision(10) << a * acos(-1) * 2 << endl;
}
