#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>

using namespace std;

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

int main() {
  int N; cin >> N;
  double pi = 3.141;

  cout << N*2*pi << endl;
  return 0;
}
