write a go solution for Description:
Let's denote a function

$$d(x,y) = \begin{cases} y - x, & \text{if } |x-y| > 1 \\ 0, & \text{if } |x-y| \leq 1 \end{cases}$$

You are given an array a consisting of n integers. You have to calculate the sum of d(ai, aj) over all pairs (i, j) such that 1 ≤ i ≤ j ≤ n.

Input Format:
The first line contains one integer n (1 ≤ n ≤ 200000) — the number of elements in a.

The second line contains n integers a1, a2, ..., an (1 ≤ ai ≤ 109) — elements of the array.

Output Format:
Print one integer — the sum of d(ai, aj) over all pairs (i, j) such that 1 ≤ i ≤ j ≤ n.

Note:
In the first example:

1. d(a1, a2) = 0;
2. d(a1, a3) = 2;
3. d(a1, a4) = 0;
4. d(a1, a5) = 2;
5. d(a2, a3) = 0;
6. d(a2, a4) = 0;
7. d(a2, a5) = 0;
8. d(a3, a4) =  - 2;
9. d(a3, a5) = 0;
10. d(a4, a5) = 2.. Output only the code with no comments, explanation, or additional text.