#include <bits/stdc++.h>
using namespace std;
using ll = int64_t;
using vll = vector<ll>;
using vvll = vector<vll>;
using pll = pair<ll, ll>;
using vpll = vector<pll>;
using vvpll = vector<vpll>;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  ;
  cout.setf(ios::fixed);
  cout.precision(32);
  ;
  int r;
  cin >> r;
  cout << 2 * 3.14159 * r << '\n';
  return 0;
}
