#include <bits/stdc++.h>
using namespace std;
long long const MOD = 1e9 + 7;
int const inf = 1e9;
long long const INF = 1e18;
inline void ios_() {
  cin.tie(0);
  ios::sync_with_stdio(false);
}
template <typename T>
int size(const T& a) {
  return (int)a.size();
}
template <typename T>
T Div(T a, T b) {
  return (a + b - 1) / b;
}
template <typename T>
bool chmin(T& a, const T& b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T>
bool chmax(T& a, const T& b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
signed main() {
  int r;
  cin >> r;
  cout << 2 * r * M_PI << endl;
  return 0;
}
