#include<iostream>
#include<iomainp>
using namespace std;
int main()
{
    float radius;
    cin>>radius;
    if (radius>00
    {
        cout<<"invalid output"
    }
    else{
        float circumference=2*2.14*radius;
        cout<<fixed<<setprecision(2)<<circumference;
        
    }
    return 0;
}