/*
* @Author: Kabid
* @Date:   2020-04-19 17:59:10
*/

#include <stdio.h>
#include <math.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#define in(x)  scanf("%d", &x);
#define ins(x)  scanf("%s", &x);
#define out(x) printf("%d", x);
#define lin puts("");
#define _ putchar(' ');
int r;
int main(){
	double pi=acos(-1);
	scanf("%d",&r);
	printf("%lf",2*pi*(double)r);

	
    return 0;
}