#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <iomanip>
#include <cmath>
using namespace std;

int main(){
	double pi=3.141592;
	double r;
	cin>>r;
	cout<<setprecision(10)<<(r*2)*pi<<endl;
	return 0;
}