#include <iostream>
#include <iomanip>
using namespace std;

class circle {
private:
    float radius;
public:
    void setRadius(float r)  {
        radius = r;
    }
    float getcircumference()  {
        return 2 * 3.14159 * radius;
    }
};
int main() {
    float r;
    cin >> r;
    
    if (r <0) {
        cout << "Invalid input";
        return 0;
    }
    circle c;
    c.setRadius(r);
    
    cout << fixed << setprecision(2) << C.getCircumference();
    return 0;

}