Today’s exercise comes from Stack Overflow:

Given an array A consisting of N integers, return the maximum sum of two numbers
whose digits add up to an equal sum. If there are not two numbers whose digits
have an equal sum, the function should return -1. For example, A = [51, 71, 17,
42] would output 93 because there are two sets of numbers with the same
digit-sum, (51, 42) with a digit-sum of 6 and (17, 71) with a digit-sum of 8,
and the first pair has the maximum sum of two numbers of 93.

Your task is to write a program to calculated the requested maximum sum. When
you are finished, you are welcome to read or run a suggested solution, or to
post your own solution or discuss the exercise in the comments below.