#include <bits/stdc++.h>
using namespace std;
static int fast_io = []() {
  std::ios::sync_with_stdio(false);
  cin.tie(nullptr);
  return 0;
}();
int main() {
  double r;
  cin >> r;
  cout << 3.14159265358979323846 * 2 * r << "\n";
  return 0;
}
