#include <bits/stdc++.h>
using namespace std;
void solve() {
  float r;
  cin >> r;
  float pi = 3.14;
  float c;
  c = 2 * pi * r;
  cout << fixed << setprecision(6) << c << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
  return 0;
}
