What design changes will improve visual appearance of the calculator

This is the HTML code for a very basic calculator. I am at beginner level on bootstrap and definitely beginner level designer. what are some changes i can do to make the visual appearance better. i have used bootstrap and tried to use some features to improve the look with minimum CSS ( i don't want to use CSS unless the same can be done with bootstrap ) . please use codepen link to see the rendering.

codepen link

HTML

<div class="row ml-5">
  <div class="form col-4 mt-5"> 
  <div class="form-group">
    <lable>First Number</lable>
    <input id="firstNum" type="text" class="form-control"/>
  </div>
  <div class="math-buttons">

    <div class="btn btn-primary col-12 mt-2">
       +
  </div>  <div class="btn btn-primary col-12 mt-2">
       -
  </div>  <div class="btn btn-primary col-12 mt-2">
        x
  </div>  <div class="btn btn-primary col-12 mt-2">
        /
  </div> 
  </div>


   <div class="form-group mt-3">
    <lable>Second Number</lable>
    <input id="secondNum" type="text" class="form-control"/>
  </div>
  <div id="btnCalculate" class="btn btn-primary col-12 mt-2">
        Calculate
  </div>


</div>
<div class="output col-4 my-auto ml-5">
  <h4 id="result">
    0.00
  </h4>
</div>
</div>

CSS

.math-buttons>.btn{
  display:inline;
  margin-left:10px;
}