#include <iostream>
#include <iomanip>
using namespace std;
class Circle{
    private:
          float radius;
    public:
          Circle(float r){
              radius=r;
          }
          float circumference(){
              return 2*(3.14159) radius;
          }
};
int main(){
    float r;
    cin>>r;
    
    if(r<=0){
        count<<"Invalid input";
        return 0;
    }
    Circle c(r);
    count<<fixed<<setpercision(2)<<
    c.circumference();
    return 0;
}