# Aligning equations in \begin{cases}

I need neither of the following:

$$\begin{cases} ab =st \\ a-b =s+t \end{cases}$$

$$\begin{cases} ab &=st \\ a-b &=s+t \end{cases}$$

Instead, I want something like this with bracket on the left:

ab=st
a-b=s+t

How to achieve this? Thanks.

$$\left\{ \begin{array}{@{} r @{{}={}} l} ab &st\\ a-b &s+t \end{array} \right.$$