John Galt requests a program to double$-$check his Gas Bill.

John$$s gas supply is read from a $4-$digit meter each month that shows the current total gas used in cubic meters. The amount of gas used that month is determined by subtracting the previous month$$s reading from the current month$$s reading. $($Note: Since the meter has only $4$ digits, the current month$$s reading may be lower than the previous month$$s reading. This implies that the meter rolled over back to $0000$ during that month.$)$

The gas company charges based on the following chart:

Create a program to determine John$$s gas bill.

What are the inputs? How are they validated? What character keys are needed?

What processing is required here?

How is his total usage determined each month? How do you account for a rollover on the meter?$\backslash$table$[[$GAS USED,RATE$],[$First $80$ cubic meters,$$10$ minimum charge$],[$Next $120$ cubic meters,$$0\mathrm{.}10$ per cubic meter$],[$Next $200$ cubic meters,$$0\mathrm{.}05$ per cubic meter$],[$Any additional cubic meters,$$0\mathrm{.}025$ per cubic meter$]]$