#include <bits/stdc++.h>
using namespace std;
void solve() {
  double pi = 3.14159;
  double rad;
  cin >> rad;
  cout << 2 * rad * pi << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
