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