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

class Cirlce {
private:
    double radius;
public:
    Circle(double r) { radius = r; }
    double circumference() {
        return 2 * 3.14159 * radius;
    }
};

int main() {
    double r;
    cin>>r;
    if( r <= 0 || r > 1000) {
        cout<<"Invalid input"<<endl;
        return 0;
    }
    Circle c(r);
    cout << fixed << setprecision(2) << c.circumference() << endl;
    return 0;
}