follow step by step! use functions! in C++ please

Question 3 Write a program that calls a function calculateSum to calculate the sum from 0 to N integers. The function calculateSu integer which represents the sum from 0 to N, inclusive. m has one parameter Nyof type integer and returns an Write another function calculateAverage that calculates an average. This function wil have two parameters: the sum and the number of items. It returns the average (of type float). The main function should be responsible for all inputs and outputs. Your functions will only calculate and return the values and NOT print anything. N is provided by the user user input must be asked for repeatedly until a non-negative integer is provided. Note: the mainO first calls calculateSum. Then, it uses the result returned by this fiunction as an argument to call calculate Average