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.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.HashSet;
import java.util.StringTokenizer;
/*
 * Java Input / Output Class using Buffered/ Input Straem
 * Created by @Hacked_
 * File Created at: Apr 16, 2019
 */
public class B {
	public static BufferedReader sc = new BufferedReader(new InputStreamReader(System.in));
	public static StringTokenizer st;
	public static PrintWriter pw = new PrintWriter(System.out);
	final static boolean debugmode = true;
	public static int k = 7; // for 10^9 + k mods.
	public static long STMOD = 1000000000 + k; // 10^9 + k

	public static void main(String[] args) throws IOException {
		HashSet<Integer> x = new HashSet<Integer>();
		int[] v = readArray(getInt());
		for(int p : v) {
			x.add(p);
		}
		if(x.size() > 3) {
			submit(-1,true);
		}
		else if (x.size() == 3) {
			int r = 0;
			int[] vals = new int[3];
			for (int p : x) {
				vals[r] = p;
				r++;
			}
			Arrays.sort(vals);
			if (vals[2] - vals[1] == vals[1]-vals[0]) {
				submit(vals[2]-vals[1],true);

			}
			else {
				submit(-1,true);
			}
		}
		else if(x.size() == 2) {
			int r = 0;
			int[] vals = new int[2];
			for (int p : x) {
				vals[r] = p;
				r++;
			}
			Arrays.sort(vals);
			//System.out.println(vals[1] + " "+vals[0]);
			if (( vals[1]-vals[0]) % 2 == 0) {
				submit((vals[1]-vals[0]) / 2,true);
			}
			else {
				submit(vals[1]-vals[0],true);
			}

		}
		else {
			submit(0,true);
		}
	}

	public static void setInputFile(String fn) throws IOException {
		sc = new BufferedReader(new FileReader(fn));
	}

	public static void setOutputFile(String fn) throws IOException {
		pw = new PrintWriter(new BufferedWriter(new FileWriter(fn)));
	}

	public static int GCD(int a, int b) {
		if (b == 0)
			return a;
		return GCD(b, a % b);
	}

	public static double log(int k, int v) {
		return Math.log(k) / Math.log(v);
	}

	public static long longpower(int a, int b) {
		long[] vals = new long[(int) (log(b, 2) + 2)];
		vals[0] = a;
		vals[1] = a * a;
		for (int i = 1; i < vals.length; i++) {
			vals[i] = vals[i - 1] * vals[i - 1];
		}
		long ans = 1;
		int cindex = 0;
		while (b != 0) {
			if (b % 2 == 1) {
				ans *= vals[cindex];
			}
			cindex += 1;
			b /= 2;
		}
		return ans;

	}

	public static void debug(String toPrint) {
		if (!debugmode) {
			return;
		}
		pw.println("[DEBUG]: " + toPrint);
	}

	public static void submit(int[] k, boolean close) {
		pw.println(Arrays.toString(k));
		if (close) {
			pw.close();
		}
	}

	public static void submit(int p, boolean close) {
		pw.println(Integer.toString(p));
		if (close) {
			pw.close();
		}
	}

	public static void submit(String k, boolean close) {
		pw.println(k);
		if (close) {
			pw.close();
		}
	}

	public static void submit(double u, boolean close) {
		pw.println(Double.toString(u));
		if (close) {
			pw.close();
		}
	}

	public static void submit(long lng, boolean close) {
		pw.println(Long.toString(lng));
		if (close) {
			pw.close();
		}

	}

	public static void submit() {
		pw.close();
	}

	public static int getInt() throws IOException {
		if (st != null && st.hasMoreTokens()) {
			return Integer.parseInt(st.nextToken());
		}
		st = new StringTokenizer(sc.readLine());
		return Integer.parseInt(st.nextToken());
	}

	public static long getLong() throws IOException {
		if (st != null && st.hasMoreTokens()) {
			return Long.parseLong(st.nextToken());
		}
		st = new StringTokenizer(sc.readLine());
		return Long.parseLong(st.nextToken());
	}

	public static double getDouble() throws IOException {
		if (st != null && st.hasMoreTokens()) {
			return Double.parseDouble(st.nextToken());
		}
		st = new StringTokenizer(sc.readLine());
		return Double.parseDouble(st.nextToken());
	}

	public static String getString() throws IOException {
		if (st != null && st.hasMoreTokens()) {
			return st.nextToken();
		}
		st = new StringTokenizer(sc.readLine());
		return st.nextToken();
	}

	public static String getLine() throws IOException {
		return sc.readLine();
	}

	public static int[][] readMatrix(int lines, int cols) throws IOException {
		int[][] matrr = new int[lines][cols];
		for (int i = 0; i < lines; i++) {
			for (int j = 0; j < cols; j++) {
				matrr[i][j] = getInt();
			}
		}
		return matrr;
	}

	public static int[] readArray(int lines) throws IOException {
		int[] ar = new int[lines];
		for (int i = 0; i < lines; i++)
			ar[i] = getInt();
		return ar;
	}

}
/*

public class CFTemplate {

}

*/