#include<iostream>
#include<iomanip>
using namespace std;
class circle{
    private:
    float radius:
    public:
    circle(float r){
        radius=r;
    }
    floar circumference(){
        return2*3.14159*radius;
    }
};
int main(){
    float r;
    cin>>r;
    if(r<=0){
        cout<<"Invalid input";
        return 0;
    }
    circle c(r);
    cout<<fixed<<setprecision(2)<<c.circumference();
    return 0;
}