#include<iostream>
#include<iomanip>
using namespace std;
 class Circle{
     private:
     float radius;
     
     public:
     Circle (float r){
         if(r>0 && r<=1000)
         radius = r;
         else
             radius =-1;
            return 2*3.141159*radius;
         
     }
 };
 int main(){
     float r;
     cin>>r;
      Circle c(r);
      float circumference = c.getCircumference();
      
      if (circumference == -1)
      cout<<"Invalid input";
      else
      cout<<fixed<<setprecision(2)<<circumference;
      return 0;
 }