// editor1
#include<iostream>
using namespace std;
class Circle
{
    private:
    double radius;
    public:
    void setRadius(double r)
    {
        if (r<=0 || r>1000)
        {
            cout<<"Invalid input";
        }
        else
        {
            radius = r;
            double circumference = 2 * 3.14159 * radius;
            circumference = round(circumference * 100)/100;
            cout<<circumference;
        }
    }
};
int main()
{
    double r;
    cin>>r;
    Circle c;
    c.setRadius(r);
    return 0;
}