#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int MOD = 1e9 + 7;
const double pi = 3.14159265358979323846;
const int inf = 2e9;
const ll INF = 1e18;
using P = pair<int, int>;
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
int main() {
  cin.tie(0), cout.tie(0);
  ios::sync_with_stdio(false);
  double r;
  cin >> r;
  cout << fixed << setprecision(10) << 2 * r * pi << endl;
}
