#include <bits/stdc++.h>
using namespace std;
const long double PI = acos(-1);
void solve() {
  double r;
  cin >> r;
  cout << setprecision(20) << 2 * PI * r << endl;
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
  return 0;
}
