css - how to define variable in less -

i trying set variables media query breakpoints in less:

my definition looks this:

@sm-screen: "(min-width: @{screen-sm-min}) , (max-width: @{screen-sm-max})"; 

so when try use following:

@media @sm-screen {    .something {        display: none;    } } 

it works ouputs double quotes compiled css ends looking like:

@media "(min-width: 768px) , (max-width: 991px)" {     .something {         display:none;     } } 

how define compiled css doesn't have double quotes wrapped round it? tried getting rid of them variable definition less won't compile following error:

parseerror: expected ')' in c:\xampp\htdocs\fund-manager\resources\assets\less\variables.less on line 11, column 14: 

i had escape string ~ character so:

@sm-screen: ~"(min-width: @{screen-sm-min}) , (max-width: @{screen-sm-max})";