#include<iostream>
using namespace std;
class Circle
{
    private:
    float radius;
    public:
    Circle(float r)
    {
        radius=r;
    }
    float gatCircumference()
    {
        return 2*3.14159*radius;
    }
};
int main()
{
    float radius_input;
    cin>>radius_input;
    if(radius_input>=1&&radius_input<=1000)
    {
        Circle circles(radius_input);
        float circumference=Circle.getCircumference(); 
        cout<<circumference<<endl;
    }
    else 
    {
        cout<<"Invalid input";
    }
    return 0;
}