Oh thaat one.

$f(x)$ is a polynomial of degree greater than or equal to $3$ .

When $f(x)$ is divided by $x-1$ , the remainder is $2$ .

When $f(x)$ is divided by $x-2$ , the remainder is $1$ .

Find the remainder when $f(x)$ is divided by $(x-1)(x-2)$ .

If the remainder is $r(x)$ , enter the value of $r(123)$ .