Changing dates format in Set analysis

[Franco2]

Hi!

I have a dataset that looks like this

Order	OrderMonth	DeliveryMonth	Amount
1	201901	201901	10
2	201901	201902	20
3	201901	201901	10
4	201901	201901	40
5	201901	201903	20
6	201902	201904	20
7	201902	201902	10
8	201902	201903	30
9	201903	201905	40
10	201903	201904	10
11	201904	201903	50
12	201905	201903	20
13	201906	201907	50
14	201906	201908	20

If i want to do a chart showing the amount of orders that were delivered the same month they were ordered i will have to put something like this:

Sum(If(OrderMonth=DeliveryMonth, Amount))

But now I have two questions:

-I want to show the sum of amounts from orders that were delivered until 1 month after. I think it should be something like 

Sum(If(OrderMonth<=DeliveryMonth-1, Amount))

-And if I find that the second question is how to change the format of the months. As you can see the 4 first digits refer to the year and the 2 last refer to the month. If I do the previous calculus it will omit the orders from december 2019 that were delivered in January 2020. I think there's a way to do it by creating a new field but I wonder if theres a way to do it directly in set analysis.

I will really appreciate your answers so much. Sorry I'm new to Qlik and I'm also not native speaker.