#include <iostream>
using namespace std;

const double Pi = 3.14;

class Yuan
{
public:
int m_r;
double clau_l()
{
    return 2*Pi*m_r;
}
};

int main() {
    Yuan c1;
    c1.m_r=10;
    cout<<c1.clau_l()<<endl;

	return 0;
}