#include "bits/stdc++.h"
//#define int long long
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
const ll INF = (1LL << 61);
ll mod = 1000000007;

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	long double x; cin >> x;
	x *= 4.0;
	cout << fixed << setprecision(25);
	cout << x << endl;
	return 0;
}