knockout.js - How to Call a KO function in input -

i new knockout js have 1 function,i call function , post return value iam unable to tyhay please me.

my code is:

<td class="right"><input type="text" data-bind="value:$root.totbilhours($index())" /></td> 

script function :

totbilhours = function(index){              var tothours = 0;              tothours = parsefloat(project.ppmbillingtsdtl()[index]['billstdhours']()) + parsefloat(project.ppmbillingtsdtl()[index]['billothours']());              return tothours.tofixed(2);           }; 

with in working fine

<td class="right" data-bind="text: $root.totbilhours($index())" ></td>