#include <iostream>
#include <iomanip>
#include <algorithm>
#include <cstdio>
#include <cstring>
#define For(i,a,b) for(i=(a); i<(b); i++)
using namespace std;

const double pi=3.1415926;

int main(){
	double r;
	cin>>r;
	cout<<2*pi*r;
	return 0;
}