Show lowest missing number in a sequence w/repeating numbers and blanks

I have a program that will be referencing this sheet so I can't have any VBA/macros or sorting involved. I just created a basic example of what I need and attached it. I can rearrange the formulas from there. I am looking for a formula in D2 that will look at column A and tell me the lowest number that is missing.It can reference column B or I don't need that all. The numbers will range from 1 to whatever in sequencial order.

There is other data that will be in this sheet, but people will be entering the number truck they placed each order (row) into (Column B). So there will be several rows/orders in the same numbered truck. There will be some that aren't in a truck yet. These trucks are transfer trucks from one facility to another and each truck is filled on different days. So for example once truck number 1 is used the next person will see they need to use truck number 2 for their orders and so on. There is a "use next" cell, but if someone uses the wrong number, in this case let's say they put in an 8, I want to say they are missing a 6 in D1.


Thanks for any help on this. I have tried several different ways, but have been unable to get the desired results.