// editor3
#include <iostream>
using namespace std;
class Circle{
    private:
    float radius;
    
    public:
    viod setRadius(float r){
        radius = r;
    }
    void calculateCircumference(){
        if(radius<=0||radius>1000){
            cout<<"Invalid input";
        }else{
            float circumference=2*3.14159*radius;
            cout<<circumference<<endl;
        }
    }
};
int main(){
    float r;
    cin>>r;
    Circle c;
    c.setRadius(r);
    c.calculateCircumference();
    return 0;
}