#include <bits/stdc++.h>
using namespace std;
struct Node {
  int node, len;
  Node() { node = len = 0; }
  Node(int node, int len) { this->node = node, this->len = len; }
};
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int r;
  cin >> r;
  cout << fixed << setprecision(5) << r * 2 * acos(-1);
}
