#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
using TP = tuple<ll, ll, ll>;
void init_io() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << setprecision(18);
}
signed main() {
  init_io();
  ll r;
  cin >> r;
  cout << r * 2 * M_PI << endl;
}
