#include<bits/stdc++.h>
#define ll  long long
#define ff first
#define ss second
#define pb push_back
#define pp pair<ll,ll>
#define maxn 1000005
#define db double
#define mod 1000000007
using namespace std;
void solve(ll ind)
{
	ll x;
	cin >> x;
	db ans = (db)22/(db)7;
	ans = ans*2*(db)x;
	cout << ans;
}
int main()
{
	ll t=1,i=1;
	ios_base::sync_with_stdio(false);
    cin.tie(NULL);	
    std::cout.precision(6);
    std::cout.setf( std::ios::fixed, std:: ios::floatfield ); 
	//cin >> t;
	//cin >> n;
	while(i<=t)
	{
		solve(i);
		i++;
	}
	return 0;
}