#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using Int = long long;
const int MOD = 1000000007;
const ll INF = numeric_limits<ll>::max();
const int inf = 1e8;
typedef std::priority_queue<int, std::vector<int>, std::greater<int> >
    IntReversePrioQueue;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  double r;
  cin >> r;
  cout << std::fixed << std::setprecision(2) << r * 2 * 3.14;
}
