#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using vi = vector<int>;
using mti = vector<vector<int>>;
using vl = vector<ll>;
using mtl = vector<vector<ll>>;
using pi = pair<int, int>;
using pl = pair<ll, ll>;
template <typename T>
using heap = priority_queue<T, vector<T>, function<bool(const T, const T)>>;
int main() {
  double r;
  cin >> r;
  cout << fixed << setprecision(10);
  cout << 2 * 3.1415926535 * r << endl;
  return 0;
}
