In this exercise, you will create a program that adds, subtracts, multiplies, or

divides two integers. The program will need to get a letter (A for addition, S for

subtraction, M for multiplication, or D for division) and two integers from the user. If

the user enters an invalid letter, the program should not ask the user for the two

integers. Instead, it should display an appropriate error message before the program

ends. If the letter is A (or a), the program should calculate and display the sum of

both integers. If the letter is S (or s), the program should display the difference

between both integers. When calculating the difference, always subtract the smaller

number from the larger one. If the letter is M (or m), the program should display the

product of both integers. If the letter is D (or d), the program should divide both

integers, always dividing the larger number by the smaller one .