#include <bits/stdc++.h>
#pragma GCC optimize("02")
using namespace std;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  double n;
  cin >> n;
  cout << fixed << setprecision(10) << M_PI * n * 2 << "\n";
  return 0;
}
