Is there a way to manipulate two bound properties upon loading them?

The title of this post doesn't really explain well. I want a placeholder property to contain the returned value of the quantity ordered, minus the quantity received, and I'm trying to do it in the controller.

qtyO = new sap.m.Input({                    type: sap.m.InputType.Number,                    placeholder: "{QtyOrdered}" - "{QtyReceived}",                    width: "100px",                    enabled: false,                    change: function (evt) {                        refreshList(evt);                    }                }),

I think what I'm trying here isn't following the strings as paths, as it returns NaN. Is there a simple way to doing this? Or would it be better done on the backend?