// editor3
#include<iostream>
#include<iomanip>
using namespace std;
int main()
{
 double r,x;
 cin>>r;
 if(r>=1&&r<=1000)
 {
     x=(2*3.14159)*r;
     cout<<fixed<<setpresicion(2)<<x;
     
 }
 else{
     cout<<"Invalid input";
 }
 return 0;
}