#include <stdio.h>
//#include <math.h>
#define MIN(x,y) (((x) > (y)) ? (y) : (x))
#define ABS(x) ((x) >= 0 ? (x) : (-(x)))
#define PI 3.14159265
int main() {
  int r;
  scanf("%d", &r);
  printf("%lf", 2.0 * (double)r * PI);
  return 0;
}