// editor3
#include <iostream>
#include <iomanip>
using namespace std;
class Circle{
    private:
    float radius;
    public:
    Circle(float r){
        radius = r;
    }
    bool isValid(){
        return(radius > 0 && radius <= 1000);
    }
};
int main(){
    float r;
    cin >> r;
    Circle c(r);
    if(!c.isValid()){
        cout << : Invalid input" << endl;
    }
    else{
        cout << fixed << setprecision(2) << c.getCircumference() << endl;
    }
    return 0;
}