#include <bits/stdc++.h>
using namespace std;
int ctoi(const char c) {
  switch (c) {
    case '0':
      return 0;
    case '1':
      return 1;
    case '2':
      return 2;
    case '3':
      return 3;
    case '4':
      return 4;
    case '5':
      return 5;
    case '6':
      return 6;
    case '7':
      return 7;
    case '8':
      return 8;
    case '9':
      return 9;
    default:
      return -1;
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  double n;
  cin >> n;
  cout << fixed << setprecision(15) << (n * 2) * M_PI << endl;
}
