Total of Sub Form in Main form

I am new to Zoho Creator and started to create forms

I am creating a Main-form and sub form

Main Form Contains Trip Details

Sub Form Containes Trip Materials

One Trip can contain many materials.

I want the total quantity of materials entered in sub form to be displayed in main form

Field Deluge Name for total in main form is : TOTMT

Field Deluge Nameto enter qty in sub form : LQTMT

So the total of LQMT is to be displayed and stored in TOTMT as and when the user types in LQTMT

I already tried using the example code in following link.But its showing error.

Needed your guidance to proceed further.