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.IOException; 
import java.io.InputStreamReader; 
import java.util.Scanner; 
import java.util.StringTokenizer; 
import java.util.*;
  
public class discounts
{ 
    //By shwetank_verma
	public static void main(String[] args) 
    { 
        FastReader sc=new FastReader(); 
       
           
                int t=1;
               // t=sc.nextInt();
              tsl:  while(t-->0){
                int n=sc.nextInt();
                int a[]=new int[n];
                for(int i=0;i<n;i++) {
                	a[i]=sc.nextInt();
                }
                boolean ok=false;
                Arrays.sort(a);
                HashSet<Integer> hs=new LinkedHashSet<>();
                for(int i:a)
                	hs.add(i);
                int b[]=new int[hs.size()];
                int c=0;
                for(int i:hs)
                	b[c++]=i;
                Arrays.sort(b);
                if(hs.size()==1)
                {
                	System.out.println(0);
                	continue;
                }
                int d=-1;
                boolean f=true;
               for(int j=0;j<=100;j++) {
            	      f=true;
            	      int val=-1;
                	  for(int i=0;i<c-1;i++) {
                		 
                		if(b[i]+j==b[c-1]||b[i]+j==b[c-1]-j) {
                			val=j;
                			break;
                		}
                		
                		
                		
                	}
                	  if(val==-1)
                		  continue;
                	  int res=val+b[0];
                	  for(int i=1;i<c;i++) {
                		  if(b[i]==res||b[i]+val==res||b[i]-val==res) {
                			  
                		  }
                		  else {
                			  f=!f;
                			  break;
                		  }
                	  }
                	  if(f) {
                	  System.out.println(j);
                	  ok=true;
                	  break;
                	  }
                	  f=false;
                }
               if(!ok)
               System.out.println(-1);
                
                }
           
            
        
    }
    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 int mod=1000000007;
    static boolean primes[]=new boolean[1000007];
    static ArrayList<Integer> b=new ArrayList<>();
    static boolean seive(int n){
        Arrays.fill(primes,true);
        primes[0]=primes[1]=false;
        for(int i=2;i*i<=n;i++){
            if(primes[i]==true){
                for(int p=i*i;p<=n;p+=i){
                    primes[p]=false;
                }
            }
        }
        if(n<1000007){
        	for(int i=2;i<=n;i++) {
        		if(primes[i])
        			b.add(i);
        	}
            return primes[n];
        }
        return false;
        
    }
    static int gcd(int a,int b){
        if(b==0)
        return a;
        return gcd(b,a%b);
    }
    static long GCD(long a,long b){
        if(b==0)
        return a;
        return GCD(b,a%b);
    }
    static ArrayList<Integer> segseive(int l,int r){
    	
    	ArrayList<Integer> isprime=new ArrayList<Integer>();
    	boolean p[]=new boolean[r-l+1];
    	Arrays.fill(p, true);
        
    	for(int i=0;b.get(i)*b.get(i)<=r;i++) {
    		int currprime=b.get(i);
    		int base=(l/currprime)*currprime;
    		if(base<l) {
    			base+=currprime;
    		}
    		for(int j=base;j<=r;j+=currprime) {
    			p[j-l]=false;
    		}
    		
    		
    		if(base==currprime) {
    			p[base-l]=true;
    		}
    		
    	}
    	for(int i=0;i<=r-l;i++) {
    		if(p[i])
    			isprime.add(i+l);
    	}
    	return isprime;
    }
      static int LowerBound(int a[], int x) { // x is the target value or key
  int l=-1,r=a.length;
  while(l+1<r) {
    int m=(l+r)>>>1;
    if(a[m]>=x) r=m;
    else l=m;
  }
  return r;
}
static int UpperBound(int a[], int x) {// x is the key or target value
    int l=-1,r=a.length;
    while(l+1<r) {
       int m=(l+r)>>>1;
       if(a[m]<=x) l=m;
       else r=m;
    }
    return l+1;
 }

  
     
}