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
from bisect import bisect_right as br
from bisect import bisect_left as bl
from collections import *
from itertools import *
import functools
import sys
import math
import random
import re
MAX = sys.maxsize
MAXN = 10**5+10
MOD = 10**9+9
def isprime(n):
    n = abs(int(n))
    if n < 2:
        return False
    if n == 2: 
        return True    
    if not n & 1: 
        return False
    for x in range(3, int(n**0.5) + 1, 2):
        if n % x == 0:
            return False
    return True

def mhd(a,b,x,y):
    return abs(a-x)+abs(b-y)

def numIN(x = " "):
    return(map(int,sys.stdin.readline().strip().split(x)))

def charIN(x= ' '):
    return(sys.stdin.readline().strip().split(x))

def dis(x,y):
    a = y[0]-x[0]
    b = x[1]-y[1]
    return (a*a+b*b)**0.5

def lgcd(a):
    g = a[0]
    for i in range(1,len(a)):
        g = math.gcd(g,a[i])
    return g

def ms(a):
    msf = -MAX
    meh = 0

    for i in range(len(a)):
        meh+=a[i]
        if msf<meh:
            msf = meh
        if meh<0:
            meh = 0
    return msf


def res(ans,t):
    print('Case #{}: {}'.format(t,ans))


n = int(input())
a = list(numIN())

for i in range(101):
    d = i
    for j in range(1,101):
        x = j
        temp = list(a)
        for k in range(n):
            #print(i,x,k)
            if temp[k]!=x and abs(temp[k]-x)==d:
                temp[k] = x
        if len(set(temp))==1:
            print(d)
            exit(0)
print(-1)