#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
#define MAXN 1000100
#define ll long long
#define inf 1000000000
double n,m, pi;
int main()
{
	pi = 3.1415926535;
	scanf("%lf",&n);
	n = n*2.0*pi;
	printf("%.6lf\n",n);
	
	return 0;
}