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.io.FileInputStream; 
import java.io.IOException; 
import java.io.InputStreamReader; 
import java.util.*; 
import java.util.StringTokenizer; 

public class Main 
{ 
    static int MAXN=1000001;
    static int spf[] = new int[MAXN]; 
    static int ans[]=new int[MAXN];
    static HashSet s=new HashSet();
	static class FastReader 
	{ 
		BufferedReader br; 
		StringTokenizer st; 

		public FastReader() 
		{ 
			br = new BufferedReader(new
					InputStreamReader(System.in)); 
		} 

		String next() 
		{ 
			while (st == null || !st.hasMoreElements()) 
			{ 
				try
				{ 
					st = new StringTokenizer(br.readLine()); 
				} 
				catch (IOException e) 
				{ 
					e.printStackTrace(); 
				} 
			} 
			return st.nextToken(); 
		} 

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

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

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

		String nextLine() 
		{ 
			String str = ""; 
			try
			{ 
				str = br.readLine(); 
			} 
			catch (IOException e) 
			{ 
				e.printStackTrace(); 
			} 
			return str; 
		} 
	} 
    static void sieves() 
    { 
        spf[1] = 1; 
        for (int i=2; i<MAXN; i++) 
            spf[i] = i; 
        for (int i=4; i<MAXN; i+=2) 
            spf[i] = 2; 
        for (long i=3; i*i<MAXN; i++) 
        { 
            if (spf[(int)i] == i) 
            { 
                for (long j=i*i; j<MAXN; j+=i) 
                    if (spf[(int)j]==j) 
                        spf[(int)j] = (int)i; 
            } 
        } 
    } 
    static void getFactorizations(int x) 
    { 
        Vector<Integer> ret = new Vector<>(); 
        HashSet w=new HashSet();
        while (x != 1 && spf[x]!=1) 
        { 
            if(!w.contains(spf[x]) && spf[x]!=1)
            {
            w.add(spf[x]);
            ans[spf[x]]++; 
            }
            x = x / spf[x]; 
        }
    } 
	public static void main(String[] args) 
	{ 
		FastReader sc=new FastReader();
	    int n=sc.nextInt();
	    int a[]=new int[n];
	    HashSet r=new HashSet();
	    for(int i=0;i<n;i++)
	    {
	    a[i]=sc.nextInt();
	    r.add(a[i]);
	    }
	    Arrays.sort(a);
	    Iterator it=r.iterator();
	    ArrayList<Integer> an=new ArrayList();
	    while(it.hasNext())
	    {
	        an.add((Integer)it.next());
	    }
	    Collections.sort(an);
	    if(r.size()==1)
	    System.out.println(0);
	    else if(r.size()==2)
	    {
	    if((an.get(1)-an.get(0))%2==0)
	    System.out.println((an.get(1)-an.get(0))/2);
	    else
	    System.out.println(an.get(1)-an.get(0));
	    }
	    else if(r.size()==3)
	    {
	        if(an.get(2)-an.get(1)==an.get(1)-an.get(0))
	        {
	        //if((an.get(2)-an.get(1))%2==0)
	        System.out.println(an.get(2)-an.get(1));
	        //else
	        //System.out.println((an.get(2)-an.get(1))/2);
	        }
	        else
	        System.out.println(-1);
	    }
	    else
	    System.out.println(-1);
	    
	    
	} 
	public static void sieven(int n) 
    {
        boolean prime[] = new boolean[n+1]; 
        for(int i=0;i<n;i++) 
            prime[i] = true; 
        for(int p = 2; p*p <=n; p++) 
        { 
            if(prime[p] == true) 
            { 
                for(int i = p*2; i <= n; i += p) 
                    prime[i] = false; 
            } 
        } 
        for(int p=2; p<=n;p++)
        if(prime[p]==true)
        s.add(p);
    } 
	public static long gcd(long a,long b)
	{
	    if(b==0)
	    return a;
	    return gcd(b,a%b);
	}
	static long power(long x, long y, int p) 
    { 
        long res = 1;      
        x = x % p;  
        while (y > 0) 
        { 
            if((y & 1)==1) 
                res = (res * x) % p; 
            y = y >> 1;  
            x = (x * x) % p;  
        } 
        return res; 
    }
    static class Pair
	{ 
		int x; 
		int y;
		public Pair(int x, int y) 
		{	 
			this.x = x; 
			this.y = y; 
		}	 
	} 
    static class Compare
	{ 
		static void compare(Pair arr[], int n) 
		{ 
			// Comparator to sort the pair according to second element 
			Arrays.sort(arr, new Comparator<Pair>() { 
				@Override public int compare(Pair p1, Pair p2) 
				{ 
					if(p1.x>p2.x)
					{
						return 1;
					}
					else if(p2.x>p1.x)
					{
						return -1;
					}
					else
					{
						return 0;
					} 
				} 
			}); 
		} 
	}
}
