#include<iostream>
#include<iomanip>
using namespace std;
    class Circle{
        private:
        float radius;
        public:
        void setRadius(float r){
            Radius=r;
 }
 float circumference(){
     return 2*3.14*radius;
     }
    };
    int main()
    {
        float r;
        cin>>r;
        if(r<=0){
            cout<<"Invalid input";
        }
        else{
            Circle c;
            c.setRadius(r);
            cout<<fixed<<setprecision(2)<<c.circumference();
            
        }
        return 0;
    }