#include<bits/stdc++.h>
#define mod 1000000007
#define ll long long int
#define ar array
#define mp make_pair
using namespace std;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(NULL);
	double r;
	cin>>r;
	cout<<fixed<<setprecision(20)<<double(2*double(22)/7*r)<<endl;
	return 0;
}