#include <bits/stdc++.h>
using namespace std;
bool compare_by_b(pair<int, int> a, pair<int, int> b) {
  if (a.second != b.second) {
    return a.second < b.second;
  } else {
    return a.first < b.first;
  }
}
int main() {
  double pi = 3.1416;
  long long r;
  cin >> r;
  cout << 2.0 * r * pi << endl;
}
