#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;
     }
     float getCircumference(){
         if(radius==-1)
         return -1;
         return 2*3.14159*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;
 }