#include <stdio.h>
#include <math.h>

int	main(void)
{
  	int	r;
  	double c;
  	int num;
  	
  	num = scanf("%d", &r);
  	if (num != 1)
  	{
  	    return (0);
  	}
  
  	c = 3.1415926535 * r * 2;
  
  	printf("%f", c);
  	return (0);
}
