// editor1
#include <iostream>
using namespace std;
class circle{
    private;
    float radius;
    public:
    bool setRadius(float r){
        if(r<0||r<1||r>1000){
            return false;
        }
        radius true;
    }
float getCircumference(){
    return 2*3.14159* radius;
}
};
int main(){
    float r;
    cin>>r;
    circle c;
    if(!c.setRadius(r)){
        cout<<"Invalid input";
    }else{
        cout<<c.getCircumference();
    }
    return 0;
}