android studio multiple checkbox return sum of check ones value?

where (text2 ) is a string=(a ,b ,c) and int val2=(1 ,2 ,3) if (a)and (b) is checked it return 13 in a textview how i make it get sum and return 4 instead, here is my code ;

AlertDialog.Builder mBuilder = new AlertDialog.Builder(Main3Activity.this);
    mBuilder.setTitle(R.string.dialog_title);
    mBuilder.setMultiChoiceItems(text2, checkedItems, new DialogInterface.OnMultiChoiceClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which, boolean isChecked) {
            if (isChecked){
                if (! mUserItems.contains(which)){
                    mUserItems.add(which);
                }
            }else {
                if (mUserItems.contains(which)){
                    mUserItems.remove(mUserItems.indexOf(which));
                }
            }
        }
    });

    mBuilder.setCancelable(false);
    mBuilder.setPositiveButton(R.string.ok_label, new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            String item = "";
            for (int i = 0; i< mUserItems.size(); i++){
                item = item + val2[mUserItems.get(i)];
                if (i != mUserItems.size() -1){
                }
            }
            mItemSelected.setText(item);
        }
    });