#include <bits/stdc++.h>
using namespace std;
double area_C(double r) {
  double S;
  S = 3.141592653589 * r;
  return S;
}
int main() {
  double r, S;
  cout << "";
  cin >> r;
  S = area_C(r);
  cout << "" << S << endl;
}
