LESS doesn't calculate length when substituting in media-query?

Question

Less

There is this piece of code:

@side-blocks-max-width: 300px;
@middle-wider-side-mult: 3;
@middle-max-width: @middle-wider-side-mult * @side-blocks-max-width;
@mobile-width: @middle-max-width;
@media all and (max-width: @mobile-width) { //что-то там }

When compiled, the latter becomes @media all and (max-width: 3 * 300px) ...instead of the expected 900px.
In another place, where it @middle-max-widthsets the block size, 900px is displayed correctly.
Tell me how to fix the situation?