#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e18;
const long long mod = 1e9 + 7;
const long long N = 2e5 + 1;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  double a;
  cin >> a;
  cout << fixed << setprecision(15) << 8 * atan(1) * a;
}
