#include <bits/stdc++.h>
const int N = 1e6 + 10;
const int mod = 1e9 + 7;
using namespace std;
void solve() {
  double r;
  cin >> r;
  double pi = acos(-1.0);
  cout << 2 * pi * r << endl;
}
int main() {
  int _ = 1;
  while (_--) solve();
  return 0;
}
