#include <iostream>
#include<iomanip>
using namespace std;
class circle
{
    private:
    float radius;
    
    public:
    circle(float r):radius(r) {}
    float getradius() const
    {
        return radius;
    }
    float getcircum() const
    {
        return 2.0f*3.14159f*radius;
    }
};
int main()
{
    float r;
    if(!(cin>>r))
    {
        cout<<"Invalid input";
        return 0;
    }
    if(r<=0.0f||r>1000.0f)
    {
        cout<<"Invalid input";
        return 0;
    }
    circle c(r);
    float circ=c.getcircum();
    cout<<fixed<<setprecision(@)<<circ;
    return 0;
}