Write a C$++$ program to calculate the sum of the integers between two numbers.

The formula to calculate this sum is:

${\displaystyle \underset{?}{\overset{?}{}}}=(\frac{n}{2})(2a+(n-1)d)$

$n$ is the number of integers to be added.

$a$ is the first number.

$d$ is the difference between each number.

Note that the program must prompt the user to enter the values of a and $n.$ For

example, if the user wants to calculate the sum between $1$ and $100,$ s$/$he would enter

$1$ for a and $100$ for n; and if the user wants to calculate the sum between $20$ and $50,$

s$/$he would enter $20$ for and $30$ for $n.$