#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
#include<stack>
#include<queue>
using namespace std;

#define read(x) scanf("%d",&x)
#define readl(x) scanf("%lld",&x)
#define ll long long 
#define ull unsigned long long

double pi=acos(-1.0);
double n; 

int main()
{
	cin>>n;
	printf("%.20lf\n",2*n*pi); 
	return 0;
} 