Proof Writing & Problem Solving · Easy problem

The statement "if $n$ is odd, then $n^2 - 1$ is divisible by $8$" is true (write $n = 2k + 1$: $n^2 - 1 = 4k(k+1)$). Its contrapositive says: if $n^2 - 1$ is NOT divisible by $8$, then $n$ is even. For how many integers $n$ with $1 \le n \le 20$ is $n^2 - 1$ NOT divisible by $8$?