Why condition does not work properly? for example enter numbers 3,4,5,6 in input(text)

JavaScript
11 
<html>
   <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
   <body> 
    
<input type="text" value=0 id='a'>
    <br>
 <input type="button" value=15 id='b'>
<br>
 <input type="button" value="" id='c'>
<script>
       
       $(document).ready(function(){$('#c').click(function(){
   
     a=document.getElementById('a').value;
    b=document.getElementById('b').value;
    if(a>b){
        alert('no')
    }
    else{
        alert('yes')
    }
    
})})
       
       </script>
   
    </body>

</html>