#include <bits/stdc++.h>
using namespace std;
const int maxn = (int)1e6 + 5;
const double pi = acos(-1.);
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  double r;
  cin >> r;
  cout << 2.000 * r * pi << endl;
}
