#include<iostream>
#include<iomanip>
using namespace std;
class circle
{
    private:
    float radius;
    public:
    bool setradius(float r)
    {
        if(r>=! && r<= 1000)
        {
            radius =r;
            return true;
        }
        return false;
    }
};
int main ()
{
    circle c;
    float r;
    cin>>r;
    if (!c,setradius(r)
    {
        cout<<"Invalid input";
        return 0;
    }
    cout<<fixed<<setprecision(2)<<c.getcircumference;
    return 0;
}