#include <bits/stdc++.h>
using namespace std;
template <typename V, typename T>
T find_num(V v, T num) {
  if (find((v).begin(), (v).end(), num) == v.end()) {
    return false;
  } else {
    return true;
  }
}
int main() {
  long long n;
  cin >> n;
  cout << 2 * n * M_PI << endl;
  return 0;
}
