arithmetic operatos

Hello Maybe it's a simple question but I could not find the symbol I need to add two fields but the "+" symbol only joins the fields but does not add the result the code is the following: $columns[] = ['label'=>'Neto','name'=>'valor_neto','type'=>'number','required'=>true]; $columns[] = ['label'=>'IVA','name'=>'valor_iva','type'=>'number','required'=>true]; $columns[] = ['label'=>'Sub Total','name'=>'sub_total','type'=>'number','formula'=>'[valor_neto] [SYMBOL?????] [valor_iva]','readonly'=>true,'required'=>true]; which is the real symbol that must be used, those of dibition, subtraction and multiplication work normally