#include <bits/stdc++.h>
using namespace std;
void PDF(double r) {
  double pi = 3.1416;
  double area;
  area = 2 * pi * r;
  cout << area << endl;
}
int main() {
  double n, i, j, l;
  cin >> n;
  PDF(n);
  return 0;
}
