#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int a, b, c, d = 0, e, f, g, i, j, k, t = 0, u = 0, w, nt;
  double res;
  string x, y;
  char ch, ch1;
  vector<int> v;
  map<int, char> mp;
  map<int, char> mp2;
  vector<string> v1;
  set<int> s;
  cin >> a;
  res = 2 * 3.1416 * double(a);
  cout << res << endl;
  return 0;
}
