#include <iostream>
#define PI 3.1415926535897
using namespace std;

    int main()
    {
    	long double radius, area, circum;

    	cin>>radius;
	
		circum = 2*PI*radius;
        std::cout << std::setprecision(11)<< circum << endl;		
		
        cout << endl;
        return 0;
    }