Andrew and Barry have created a locking mechanism for a safe in their castle.
It requires 8 inputs, either up or down, to be in the correct order.
They decide to flip a coin to determine the code to open it.
Heads will represent up and Tails will represent down.

Andrew flips HHTT in order. He writes it down on a paper and writes “1st.”
Barry flips HTHT in order. He writes it down on a paper and writes “2nd.”
The 8 inputs for the safe are Andrew’s then Barry’s code (HHTTHTHT).

They decide that in case one of them is absent, the other should still be able to deduce the full 8 input code for the safe.
By following a certain rule, they compare their two papers and come up with a 4 letter code to write next to the safe (consisting of H and T).
They also write H (Up) and T (Down) to remember how the switches go.

Each of them takes their own paper and leaves.

Caleb, a genius, raids the castle.
Andrew hears him and swallows his paper before Caleb comes and kills him.
Caleb then kills Barry and takes his paper.
He heads to the safe.
Using Barry’s paper and the code written next to the safe, he is able to open the safe and steal their gold.

What was the code written next to the safe?

(Hint: If he only had Andrew’s paper instead of Barry’s, he’d still be able to open it)