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

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

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.

Examples

Input


6
1 4 4 7 4 1


Output


3


Input


5
2 2 5 2 5


Output


3


Input


4
1 3 3 7


Output


-1


Input


2
2 8


Output


3
import java.io.*;
import java.util.*;

public class B {

    static class InputReader {
        BufferedReader reader;
        StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public float nextFloat() {
            return Float.parseFloat(next());
        }

    }

    static InputReader r = new InputReader(System.in);
    static PrintWriter pw = new PrintWriter(System.out);

    public static void main(String[] args) {
        int n = r.nextInt();
        HashSet<Integer> set = new HashSet<Integer>();
        for(int i = 1; i <= n; i++){
            set.add(r.nextInt());
        }
        if(set.size() > 3){
            pw.println(-1);
            pw.close();
            return;
        } else if(set.size() == 1){
            pw.println(0);
            pw.close();
            return;
        } else{
            ArrayList<Integer> list = new ArrayList<Integer>(set);
            Collections.sort(list);
            if(list.size() == 2){
                int nnn = list.get(1) - list.get(0);
                if(nnn%2 == 0){
                    pw.println(nnn/2);
                } else{
                    pw.println(nnn);
                }
                pw.close();
                return;
            } else{
                if(list.get(2) - list.get(1) == list.get(1) - list.get(0)){
                    pw.println(list.get(1) - list.get(0));
                } else{
                    pw.println(-1);
                }
            }
        }
        pw.close();
    }
}