write a go solution for Description:
You are given a sequence a_1,a_2,...,a_n consisting of n integers.

You can choose any non-negative integer D (i.e. D>=0), and for each a_i you can:

- add D (only once), i. e. perform a_i:=a_i+D, or
- subtract D (only once), i. e. perform a_i:=a_i-D, or
- leave the value of a_i unchanged.

It is possible that after an operation the value a_i becomes negative.

Your goal is to choose such minimum non-negative integer D and perform changes in such a way, that all a_i are equal (i.e. a_1=a_2=...=a_n).

Print the required D or, if it is impossible to choose such value D, print -1.

For example, for array [2,8] the value D=3 is minimum possible because you can obtain the array [5,5] if you will add D to 2 and subtract D from 8. And for array [1,4,7,7] the value D=3 is also minimum possible. You can add it to 1 and subtract it from 7 and obtain the array [4,4,4,4].

Input Format:
The first line of the input contains one integer n (1<=n<=100) — the number of elements in a.

The second line of the input contains n integers a_1,a_2,...,a_n (1<=a_i<=100) — the sequence a.

Output Format:
Print one integer — the minimum non-negative integer value D such that if you add this value to some a_i, subtract this value from some a_i and leave some a_i without changes, all obtained values become equal.

If it is impossible to choose such value D, print -1.

Note:
None. Output only the code with no comments, explanation, or additional text.