#include <iostream>
#include <math.h>
using namespace std;
 
int main() {
	// your code goes here
	int r;
	cin >> r;
   
    long double res = 2 * r ;
	
	cout << fixed << setprecision(20) 
	     << res*3.14159265358979323846 << endl;
	
	return 0;
}