#include <stdio.h>
#include <stdlib.h>

#define PI 3.1415926535

int main(void){
  int R;
  double result;
  if(scanf("%d",&R) != 1){
    printf("***SCANF ERROR***\n");
    exit(1);    
  }
  result=2*PI*(double)R;

  printf("%.10f\n",result);
  
  return 0;
}
