How to implement line-through using css to custom checkbox?

    ---------------------------------HTML code----------------------------------------  
       <label class="round">
            <input type="checkbox" id="round-checkbox" class="check-btn">
            <span></span>
        </label>
            <label for="round-checkbox" id="task-text" >Hello Smple TEXT</label> 

-----------------------------------------CSS code-----------------------------------------------

  .round input:checked+label{
    text-decoration: line-through;
}

I tried the above approach but it is not working...