#include <bits/stdc++.h>
using namespace std;
const long long INF = 1LL << 60;
using namespace std;
using ll = long long;
using P = pair<int, int>;
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
using Graph = vector<vector<int>>;
int main() {
  ll pi = 314159;
  ll R;
  cin >> R;
  ll ans = R * pi * 2;
  double a = (double)(ans) / 100.0;
  cout << a << endl;
  return 0;
}
