/*************************************************************************
	> File Name: A.cpp
	> Author: Knowledge_llz
	> Mail: 925538513@qq.com 
	> Created Time: 2020/4/19 19:57:23
 ************************************************************************/

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cmath>
#include<cstring>
#include<algorithm>
#define For(i,a,b) for(register int i=(a);i<=(b);++i)
using namespace std;
const double pi=acos(-1.0);
int read(){
	char x=getchar(); int u=0,fg=0;
	while(!isdigit(x)){ if(x=='-') fg=1; x=getchar(); }
	while(isdigit(x)){ u=(u<<3)+(u<<1)+(x^48); x=getchar(); }
	return fg?-u:u;
}
int main()
{
	int r;
	scanf("%d",&r);
	printf("%.18lf\n",r*pi*2);
	return 0;
}
