#include <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < (n); ++i)

using namespace std;

using ll = long long;
using P = pair<int, int>;

int main() {
  int r;
  cin >> r;

  double res = (double) 2 * r * M_PI;

  cout << fixed << setprecision(10) << res << endl;

  return 0;
}
