#include<iostream>
#include<iomanip>
using namespace std;
class Circle:{
private:
float radius;
public:
void setRadius(float r){
    radius=r;
    }
void displayCircumference(){
    if(radius<=0|| radius>100){
        cout<<"Invalid input"<endl;
        
    }else{
        float circumference=2 * 3.14159 * radius;
        cout<<fixes<<setprecision(2)<<circumference<<endl;
    }
}
};
int main(){
    float r;
    cin>>r;
    Circlec;
    c.setRadius(r);
    c,displayCircumference();
}