#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#define MAX(a, b) ((a) > (b) ? (a) : (b))
#define PI 3.14159265359

int main(void)
{
double R;
  

  scanf("%lf", &R);
  printf("%lf\n", R*2*(double)PI);
  
  
  return 0;
}
