#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B>
bool chmin(A &a, const B &b) {
  return b < a && (a = b, true);
}
template <typename A, typename B>
bool chmax(A &a, const B &b) {
  return a < b && (a = b, true);
}
int main() {
  double r;
  cin >> r;
  cout << 2 * r * 3.14159265 << '\n';
  return 0;
}
