Given a positive integer return result of product - sum.
Integer (1>=n <=10^6)
For example:
Input : 123456
product = 720
sum = 21
Output : 699

Given an array a of non-negative integers, find the number of distinct pairs of integers for which the sum is equal to k.
For example : k =8 and a =[2,3,6,2,8], the output should be 1 as [2,6] is the only distinct unique pair.