#include <bits/stdc++.h>
using namespace std;
void solve() {
  long double r;
  cin >> r;
  long double ans = M_PI * 2.0 * r;
  cout << setprecision(15) << ans << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t;
  t = 1;
  while (t--) solve();
  return 0;
}
