#include <bits/stdc++.h>
using namespace std;
void read_and_solve() {
  double r = ({
    double a;
    scanf("%lf", &a);
    a;
  });
  cout << 2.0 * 2 * acos(0.0) * r << endl;
}
int main() {
  read_and_solve();
  return 0;
}
