//#include<bits/stdc++.h>
#include<iostream>
#include<cstring>
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
typedef long long ll;

int main(){
	float pi=3.141592;
	float r;
	cin>>r;
	cout<<r*2*pi<<endl;

	return 0;

}