#include<iostream>
using namespace std;
class Circle{
    private:
    float radius;
    public:
    void setRadius(float r){
        radius = r;
    }
    void calculateCircumference(){
        if(radius<=0 || radius>1000){
            cout<<"Invalid input"<<endl;
        }
        else{
            float circumference = 2*3.14159*radius;
            cout<<fixed<<setprecision(2)<<circumference<<endl;
        }
    }
};
int main(){
    float r;
    cin>>r;
    
    Circle c1;
    c1.setRadius(r);
    c1.calculateCircumference();
    return 0;
}