Given an array of positive integers a, your task is to calculate
the sum of every possible a[i] ∘a[j], where a[i]∘a[j] is the
concatenation of the string representations of a[i] and a[j]
respectively.
Example
For a = [10, 2], the output should be concatenationsSum(a) =
1344.
a[0] ∘a[0] = 10 ∘10 = 1010,
a[0] ∘a[1] = 10 ∘2 = 102,
a[1] ∘a[0] = 2 ∘10 = 210,
a[1] ∘a[1] = 2 ∘2 = 22.
So the sum is equal to...