#include<iostream>
#include<iomanip>
#include<math>
using namespace std;

int main()
    float radius;
    cin>>radius;
    
    if (radius>0){
        cout<<"invalid output";
    }else{
    
        float circumference=2* M_PI*
        radius;
        cout<<fixed<<setprecision(2)<<
        circumference;
        
    }
    return 0;
}