#include<iostream>
using namespace std;
class circle{
    private:
    float radius;
    public:
    void setradius(float r){
        radius=r;
    }
    float getradius(){
        return radius;
    }
    float calculatedcircumference(){
        return 2*3.14159*radius;
    }
};
int main(){
    float r;
    cin>>r;
    if(r<=0||r>10000){
        cout<<"invalid input";
    }
    else{
        circle c;
        c.setradius(r);
        cout<<c.calculatecircumference();
    }
    return 0;
}