#include <bits/stdc++.h>
using namespace std;
using ll = long long;
ll mod = 1000000007;
const ll INF = 1LL << 60;
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
int main() {
  double r;
  cin >> r;
  cout << setprecision(10) << 2 * r * 3.14159265 << endl;
}
