What's inside it?

Let us first consider for a smaller square(let's say for triangle having co-ordinates: $(0,0), (0, 5) and (5,0)$ )

We observe that there are a total of $\frac{(n-2)(n-1)}{2}=\boxed{6}$ integral points, let us call this equation $1$

now we know the pattern and the foramula required then it can easily be done for $(0,0), (0, 21) and (21,0)$

substituting $n=21$ in $1$

we get: $\frac{(19)(20)}{2}=\boxed{190}$