#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using Graph = vector<vector<int>>;
const double pi = acos(-1);
int main() {
  double r;
  double ans = 2 * pi * r;
  cout << fixed << setprecision(10);
  cout << ans << endl;
  return 0;
}
