#include <bits/stdc++.h>
using namespace std;
void YN(bool flg) {
  if (flg)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
void Yn(bool flg) {
  if (flg)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
void yn(bool flg) {
  if (flg)
    cout << "yes" << endl;
  else
    cout << "no" << endl;
}
const double PI = acos(-1.0);
int main() {
  double r;
  cin >> r;
  double ans = 2 * r * PI;
  printf("%.10f", ans);
  return 0;
}
