#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <queue>
#include <cmath>
#define S 10000005
#define SS 2000
#define SSS 300
#define ll long long
#define Pi 3.141592635 
using namespace std;
long double R;
long double C;
int main()
{
	scanf("%llf",&R);
	C=2*R*Pi;
	printf("%Lf",C);
} 
