I have two fields in a class, in a third field i need the multiplication result of the two fields declared before.

For example:

```
_columns = {
            'Item' : fields.integer('Items'),
            'Fecha': fields.date('Fecha del Documento', required=True, select=True),
            'Codigo Arancelario' : fields.integer('Codigo Arancelario'),
            'Descripcion Arancelaria' : fields.char('Descripcion Arancelaria', size=42, required = True, translate = True), 
            'Especificaciones Tecnicas' : fields.char('Especificaciones Tecnicas', size=60, required = True, translate = True), 
            'Cantidad' : fields.float('Cantidad'), 'Unidad de Medida': fields.many2one('product.uom', 'Unidad de Medida'),
            'Precio Unitario Declarado' : fields.float('Precio Unitario Declarado'), 'Moneda' : fields.many2one('res.currency', 'Moneda'),
            'Valor En Divisas' : Result of the multiplication of 'Precio Unitario Declarado' * 'Cantidad',
            'notas' : fields.text('Notas'),
            }

```

Should i use a function field for such a simple calculation?

Is there a simpler way to accomplish this?