#include <stdio.h>

int main(void){

	int R=0;
	double enshuritu=0;
  	double kekka=0;
  	
  	enshuritu=3.14;
  
	scanf("%d",&R);
  	kekka=R*2*enshuritu;
  	
  	printf("%f",kekka);
  
  	return 0;
  
}