// editor4
#include <iostream>
#include <cmath>
using namespace std;

class Circle
{
    private: float r;
    public:
    Circle(float r)
    {
        this -> r = r;
    }
    void display()
    {
        float c = 2*3.14159*r;
        cout << fixed << setprecision(2) << c << endl;
    }
};

int main()
{
    float r;
    cin >> r;
    
    if(r>=1 && r<= 1000)
    {
        Circle obj(r);
        obj.display();
    }
    else
    {
        cout << "Invalid input";
    }
    return 0;
}