4
Your question can be made precise in a variety of different ways, but you first need to know logic and Peano Arithmetic (PA). (You can skip these links if you understand the rest of this post.)
$\def\nn{\mathbb{N}}$
$\def\t#1{\text{#1}}$
$\def\place#1{\,\boxed{#1}\,}$
$\def\code#1{\underline{#1}}...